This is exactly what we’re doing with PLs. But instead of just a power duration curve, we’re taking into account repeatability, muscular endurance, and other things.
I think the confusion point might be that workouts are made in percent of FTP, so you think that means we’re prescribing based on that.
But you can do it with pure wattage too. If we think your threshold/vo2 max ratio is lower, we might prescribe you at 117% VO2 intervals.
FTP - 300
117% VO2 - 351
Where someone else might be a ratio of 120%.
FTP - 300
120% VO2 - 360
You could say “This doesn’t work because it’s a percentage”, but the outcome is pure wattage.
You don’t want to do this with just a power curve because it doesn’t take repeatability into context, which is its own thing that has to be considered. We thought about just using a power curve years ago but realized it would lead to a lot of bad outcomes.
So PLs take into account all of that (you can think of it in terms of pure wattage + repeatability + muscular endurance) to get the correct workouts for you.
I think your point would be valid if we took FTP then just pushed people into 3x5min at 120% FTP. That would mean there was a static relationship and we weren’t pushing people through progressions.
If that were the case, doing a power curve duration would be a logical next step. But with PLs we’ve leapfrogged that and have a better solution that takes more things into consideration.