I disagree with your statement regarding wheel stiffness; it's actually proportional to the amount of tension in the wheel, not the amount of material in the spokes. You can make a very flexible wheel with thick spokes and insufficient tension. Of course, heavier spokes can handle higher tension, but the tension limit is typically dictated by what the rim can handle, not the spokes. Rim stiffness is also a huge factor in the overall stiffness of a wheel. There are many factory wheels that use extremely stiff rims with very low spoke counts. They also generally use heavier spokes at very high tensions. These wheels are engineered as a unit with the component parts designed to work together. Those of us who build wheels with off-the-shelf parts don't have the luxury of that custom-engineering; we have to take what's available to us and mix and match to suit a particular need. As you correctly pointed out, it's entirely possible to mismatch the wheel to the rider and the result is wheels that don't last.
Super Spokes are a special purpose item and are so ridiculously expensive that I wouldn't consider them for anything that I would ride. I don't use Ti spokes either, for similar reasons.
I also differ with your explanation of how butted spokes work. Spokes don't fail because they're over-stressed or not strong enough, they fail from fatigue, typically at the elbow. I don't think I've ever seen a spoke break at the threads, or if I have it was such a rare occurrence that I can't recall it. Failures in the center of the spoke are typically caused by physical damage, such as a chain over-shifting into the spokes.
Torque from disc brakes can be more of a factor, but primarily on front wheels where braking forces are highest.
Fatigue occurs when spokes go slack under vertical load, then regain tension as the load is removed. This can be caused by insufficient tension in the spokes or by spokes that are too stiff to absorb the small deflections in the rim under load. How much rim movement can be absorbed is dependent on how much the spoke stretch when tensioned. If you compare a 2.0mm straight-gauge spoke with a Laser/Revolution, the latter has only 57% of the cross-sectional area and will stretch nearly twice as much the straight-gauge spoke under the same tension. That means that it can also absorb nearly twice as much rim deflection, reducing the likelihood that it will experience fatigue-inducing slack cycles in normal use. It's this increased ability to absorb rim deflection that makes wheels build with butted spokes more durable, all else being equal.
That said, there are practical limits to how light a wheel can be and still support the rider adequately over the long haul. With the light rims I use on the road, my wheels are getting close to that practical limit. I don't have durability issues, but I'm pretty easy on wheels and I wouldn't recommend my exact builds to riders who are significantly heavier or more abusive to their equipment. My off-road wheels are more rugged and perhaps more suitable for the average road rider of my weight.
In regards to the strength of the wheel I think it's both, the thickness of the steel and the tension. I've seen a lot wheels fail that the spokes were not strong enough to hold the rider after a period of time but the riders wanted the lightest wheel they could make, some of those were high spoke count and some were low spoke counts. Obviously I can't tell you what the tension was on all those wheels but I doubt all of them were tensioned incorrectly, and if you over tension that leads to the eyelet area of the rim to crack. If it was only tension that causes issues then touring and tandem bikes would be built using the lightest spokes made, but we find this isn't the case, touring bikes, tandems and now e-bikes are built with the thickest butted spokes (they use to be built with just straight gauge but time showed that wisdom was flawed), this is one of the reasons DT came out with Alpine III's. And of course touring and tandem wheels are built with high spoke count rims that use a stronger heavier rim. But I doubt our original poster needs Alpine III spokes, but the point is he shouldn't go with the lightest spoke on the market. If again it was all about tension then there would be no need for different thicknesses of spokes that manufactures make, they would just make one thickness of spoke, the lightest because that's what people want, and go with that because tension would take care of any problems of load bearing down on the wheel.
I guess what I really find strange is that straight gauge spokes are recommended for those who ride disk brake bikes and even tandems, but supposedly we all talk about dble butted being better than straight. I'm confused as to why Sapim recommends those spokes for those applications.
This wheel builder discusses this:
https://whosatthewheel.com/2014/12/27/my-views-on-spokes/
And yes there are companies building heavier wheels for low spoke count reasons, I have a set, but look what happened, they had to make the rim quite a bit heavier which makes a rim that isn't any lighter then a rim that is lighter with a few more spokes! Also with low spoke count wheels if you break a spoke the wheel tacos and you have to walk home, you can't re-adjust the other nearby spokes to compensate for the damaged spoke, the wheel is toast. I was riding one of my bikes with 36 spoke wheels and a stick hit the spokes and snapped a spoke, all I did was twist the broken spoke around another spoke so it would hit anything as I rode it home, AND I didn't even have to re-adjust the surrounding spokes (I couldn't anyways because I didn't have a spoke tool), the rim was out of alignment just enough to contact the brake pad so I opened the brake calipers up and rode it home with no problems. I could have never done that with a low spoke count rim.
In my lousy opinion I don't see the rationale behind an average everyday rider who's not racing to be riding on low spoke count wheels, they do so because it's the fad, it's what comes with most new bikes as well, but with the possibility of breaking a spoke causing the wheel to taco and causing the rider to crash which could injure the rider and damage the bike itself, to me it just doesn't make sense. And to say that breaking a spoke a spoke on a low spoke count wheel will never happen...BS, it's happens, my mechanics at my bike shop have see the after effects and I've even seen it happen on the street once, I've seen what it did to the rider and the bike going about 22 mph since I was behind him doing the same speed. The bike was toast because it snapped the CF fork and the CF handlebar upon impact, and cracked the head tube (not sure if that was from the wheel tacoing or the impact, I think it was from the wheel failure but there is no way to tell for sure); the rider dislocated his shoulder, and broke a finger and jammed the wrist of the same hand, he also destroyed his helmet but his head was fine thanks to the helmet. I never did find out why that spoke broke but the guy riding it weighed around 220 to 240 pounds. Actually I think the guy came out of the crash in pretty good shape, something like that could have been a lot more serious.
Failure at the threads I don't think is very common, I can't recall ever seeing that happen personally but maybe I did but thought the spoke broke at the elbow? but evidently it does happen because it's discussed on the internet; see:
http://www.wheelfanatyk.com/blog/for-spoke-nerds-only/ Also on that site the builder talks about spokes lasting 100,000 miles, how can that be if the rims only last 30,000 miles? Because if the spokes are tensioned correctly initially during the build process and then detensioned correctly after the rim is worn out you can reuse those spokes on a new rim. So while spokes do fatigue the fatigue issue isn't a big deal if the wheel is build and later unbuilt correctly. There is also a forum discussion about threads breaking with Sapim spokes, see:
https://www.velocipedesalon.com/forum/f40/sapim-spokes-breaking-threads-26340.html Once the original poster posted others later chimed in with the same problem if you continue to read through the discussion which has 5 pages to it. So it can and does happen.
Like you, I'm a smooth rider so I don't stress my components like a lot of other riders who are not smooth riders, so I've never had a wheel fail from riding, and only one spoke ever broke and that was due to a stick. My smoothness may be one of the reasons my chains last 12 to 15,000 miles though I do keep them clean and lubed so it's probably a combination of those things.