Sure, but not every improvement is an increase in FTP
You can feel it. If you go out and do 8-10min intervals. Start ~10W below threshold then add ~5W for each interval. You’ll notice when you get above threshold.
And I agreed with you. But I’m not talking about a 20min test here. Just that even without doing a ‘test’, if you have some experience and a well trained RPE you can go out and feel when you’re at FTP.
Without looking at power I’m pretty confident I could do a 4x10m threshold workout and get within 5W of what I know my FTP is.
I haven’t done an FTP test in over a year but I’ve still closely tracked my FTP improvements the whole time.
In phases/training plans with steady state threshold work, that’s rather easy. However, I find it necessary to recalibrate my RPE meter regularly. Similar to zeroing a power meter.