I think it really helps to think of FTP, Critical Power, and heavy/severe domain border as essentially the same thing. All 3 represent moving from stable to unstable physiology. The ‘upper threshold’ is the dividing line between:
- stable oxygen consumption vs continuously increases in breathing & oxygen consumption until you hit max (vo2max)
- stable heart rate vs increasing to HRmax
- stable blood lactate generation vs increasing blood lactate
Those are slow increases if you are just above the dividing line. If you push a lot higher power, then those are a lot quicker increases. If you push enough power, you can blow thru your anaerobic capacity and be forced to tap out possibly before those reach max values (think of a 30-sec or 1-min all-out effort).
How long you can hold ftp is a separate question. How long you can hold ftp can be trained. With more and more training, you can push it out longer and longer. The CP crowd says you can sustain it for around 20-40 minutes. The FTP crowd says you can sustain it for around 30-70 minutes.
This is within reach of an amateur cyclist with average genetics, I’m living proof. It only requires motivation, lots and lots of motivation, and lots and lots of training, to ride around threshold power and do it for 40-50+ minutes.