It would be great to be able to see the highest workout levels that you have achieved over the course of a season or all time. Similar to how you can see your highest power over a range of dates, it would be great to be able to see that I’ve just done a level X.X VO2/threshold/sweetspot etc. workout and how that fits in to workouts I have completed in the past.
Itd be nice to know (mainly for a feel good factor) but it wouldn’t really give you anything useful IMO. You might have achieved a PL of 10 with an FTP of 150w, but only a PL of 4.5 with an FTP of 340w arguably the latter is the better performance and will certainly have made you faster.
I don’t think that big swings in FTP are the reality for most. I would guess that a lot of people are in a similar situation where FTP will either fluctuate through a season or only have a small increase. I think there is a use case for race planning too. Along the lines of - I was at 300W FTP last year and going in to a target race I had reached a threshold workout level 6, this year I’m at a similar FTP but I’ve recently achieved a Threshold level 9 so I can push up my pacing target by a few %.
I think there is a benefit to the gamification approach as well, as you said, there’s a feel good factor to hitting a workout level that you haven’t reached before. I would argue that a level 8 at 200W FTP feels largely the same as a level 8 at 300W (“it never gets easier, you just go faster”). For someone coming back from injury or back in to training after a break, it could be a good motivation.