Actually, that sort of argument doesn't work. Both will last as long as the owner wants them to, and in fact, can easily outlast the owner. There are zero technical reasons why CF products won't last as long as bikes. If you want, I can show you space hardware, made of CF that experiences loads orders of magnitude greater than a bike that will still be intact long after we're dead.
Ride quality is also an invalid argument for several reasons:
- Ride quality is, by far, determined by tire inflation pressure, seat, wheelbase, and fit. Full stop.
- Such an argument implies that steel, CF, aluminum, and magnesium have some inherent ride quality. This is completely wrong. If it were so, there would be a lot of materials engineers who would be suddenly unemployed. That such an argument is a load was proven by a magazine years ago that did a blind test wherein riders were supposed to determine what material the frame they were riding was made of (again, they could not see the frame.). The net result was that riders couldn't reliably discern what material composed a given frame.
This is where information for new cyclists...and even experienced cyclists...goes all pear shaped, i.e. when experienced folks continue to present unfactual information or myths as fact. It is for that reason that the whole, stupid "steel is real" BS continues. Now, before you chime in, yes I've ridden steel frames. Lots of 'em, thank you.
One thing is for damned sure: don't go to you LBS expecting to learn something factual about frame materials.