A lot of professional software has a hard plateau. One problem is that FCP 7 stopped working with macOS High Sierra.

Even if the software plateaus, they need to keep up with the incremental changes around them; newer codecs, integrations (software and hardware), and other tools.

I'm going to take a guess and say there were movies cut on Final Cut Pro 6, 5, 4, etc. It takes a lot of skill to be a good editor but I would bet most professional editors could edit a movie on an iPhone with iMovie if they had to.

Selling essentially feature complete software as a subscription makes that even harder to swallow.