FCP 7 is a very powerful tool that many editors used for many years. It’s not like they edited Parasite in iMovie.
If anything, this shows how important good tools are for a professional. The filmmakers presumably didn’t “upgrade” to FCP X because part of experience is mastering your tools and knowing what you can get out of them. I’m sure the FCP codebase was ancient and becoming difficult to maintain (it used to run on Mac OS 9!) but the wholesale rewrite they did meant that professionals now had to throw out a lot of their tool-specific knowledge learn a brand new editor, whether that editor was Apple’s initially buggy and feature-incomplete one or Adobe Premier. Premier had its own issues, but it was already proven usable in industry, had been around long enough that switching meant you weren’t as much of a Guinea pig as you were going with FCP X, and unlike FCP 7 was actively maintained.
If Apple had gone for a modular rewrite of components of FCP 7, gradually modernizing the code and the interface, they could have avoided alienating a lot of their user base. I haven’t used FCP X so I can’t say if the end result was “worth it” but FCP 7 (and the prior versions) were such a dominant force in the industry, and now FCP X is just another player.