A few thoughts:
- FTP represents a power a bit above lab tested OBLA or MLSS so your blood lactate should be rising throughout an hour ridden at FTP.
- Even if you ride below OBLA or MLSS you'll eventually fatigue even if you continue to fuel as you ride. IOW we don't understand all the reasons athletes fatigue or why their power falls off even when they ride at apparently sustainable levels.
- Power for shorter durations such as 5, 10, 15 or even 45 to 60 minute MMP are predicted fairly well using Monod and Scherrer's two part model based on AWC and CP but their model doesn't account for any fatigue and their CP term (which BTW is very close to field tested FTP) should by their definition be sustainable for a very long time. In practice it is not and actual rider's MMP curves fall off quite a bit from what their model suggests for very long durations such as efforts longer than 90 minutes or so.
So basically FTP is max sustainable power for roughly an hour on an ideal day (not day in, day out reliable hour power) but that power is above an athlete's OBLA/MLSS so it's not surprising that we struggle to complete a full hour at FTP as we're playing a deficit spending game in blood lactate terms when we ride that hard. But even if we did ride a bit easier we couldn't sustain it indefinitely. There's also the question of overall conditioning and fatigue resistance. IOW, a rider might be able to punch out some strong 20 to 30 minute efforts but find their power drops off quite a bit by an hour just because they lack the base and basic conditioning to sustain their efforts. Longer efforts, more base and increased fatigue resistance not to mention plain old riding focus to sustain hard efforts for a full hour can help a lot there.
-Dave