In my experience, people do hack game engines all the time for game specific optimizations, fixes and special features. At least in the bigger AA/AAA games I've seen. We tried to sell our plugin as binary only (C++), but basically everybody wanted the source code to hack and extend it. So people wouldn't be happy with MPL.

I agree for other things, e.g. webdev, I never had to hack Django or Vue.js.

The game industry is notorious for using, but not contributing back to open source. I don't want to know how often the same bug in recast/detour has been fix in different games, and never upstreamed.