Fwiw, I’ve tried to use OpenShot and it sucked hard. Granted, this was on Windows and it was several years ago (3~4yrs), so it may have gotten better (or it might be way better on Linux), but I ended up switching to davinci resolve because OpenShot literally couldn’t deal with anything above 720p without crashing. I couldn’t complete any university assignments with it. Even just clipping the ends of a video was causing it to crash. Unless OpenShot has gotten significantly better on windows, that’s a good example of a place where a pro-quality FOSS alternative doesn’t really exist.
You say they’re viable, but most people don’t want to make their own engine to make a game. Most people wanna make a game, not a game engine. Furthermore, while I honestly believe that the FOSS community could easily match commercial projects, doing so requires people to acknowledge the software’s shortcomings, talk to the professional users about what kinda UI they want, and so on. That means you can’t throw lines at them like “user inexperience” or “just learn a new program”. They’re professionals, having to learn a new program cuts into their income. Learning new programs means they aren’t making things, which means they aren’t making money.
I know I’m kinda rambling a bit and may not be super coherent (if so, I apologize, I’m running on low sleep), but I’m not trying to be a doomer or say “that’s impossible”; I think what I am trying to say is that I don’t think the FOSS community currently has all the necessary software features or ease of access for professionals.