No magic bullet - you've still got to do the training!
FTP is just one sustainable power measurement - and I beg to differ; it changes a lot. What you are describing is Tempo pace, and you don't waht to be doing a lot of that as it will wipe you out. If you are talking about training LT it s 85-95% MHR not 85-95% of FTP. Threshold training as a % of FTP is 95-105%.
The levels described by Allen & Coggan are
Tempo: avg power 76-90% of FTP or approx 84-94% of avg HR
Lactate Threshold: 91-105% FTP or approx 95 - 105% of avg HR
Tempo level workouts are possible on consecutive days if duration is not excessive and dietary carb intake is adequate. LT workouts are performed when rider is sufficiently rested and not normally on consecutive days although possible.
FTP does not change a lot except for new riders who can make large jumps relatively quickly. The more trained you are the more difficult it is to improve or sustain FTP.
I don't see why we need experts to have a debate - what a strange suggestion. Take that to its conclusion and there would be no forum.
I very much respect the contribution Bill makes to this forum and appreciate his experience on many threads. It is evident that Bill doesn't need a power meter due to how well he understands his body after many years of successful training and racing.
I'm not sure i need one either but wouldn't mind giving it a try someday...