They could have very well used lots or reusable parts of GCC. It was possible, and were many projects trying to change GCC.

gcc was specifically designed to not have many reusable parts to avoid people ripping them out. modularity only happened after LLVM and clang specifically started eating its lunch.

> LLVM and clang specifically started eating its lunch

I don't know how you can eat GCC's lunch. I feel like in this analogy GCC eats from a communal trough and is very happy to share. A company that sells licenses can lose customers, the GNU project just wants the world to benefit. GCC is still ubiquitous and has plenty of developers and mindshare, what more could they want?

Basically most of the embedded folks that used GCC forks are now on clang, and if in the old days they were breaching GPL, nowadays they are fully compilant, so not upstreaming is not even a legal issue any longer.

Fair enough. I guess you're talking about microcontrollers rather than lightweight CPUs, at least in my experience it's still standard to use GCC on the sort of dinky ARM-based devices my company produces.

I'll note that GPL never says anything about upstreaming. It can certainly make life easier if you want to keep your patches up to date and hand off maintenance responsibility, but it suffices to make a written offer of source code under the same license (on a conventional medium) to your customers to comply with any GPL version. You're never obliged to upstream or even publish your code.

Sure, but they didn't even do that most of the time, with MIT that isn't even part of the conversation.

Most certainly not. The modularity was not great, but BIG chunks of code could be copied. I did that myself.