> Also wasn't the D compiler proprietary and a paid product for a long time after its initial release?

The backend for the DMD compiler was not fully open source for a number of years. That's because Symantec owned some of the code and they were not willing to let it be relicensed. They did allow that in 2017. It was never a paid product AFAIK.

Overall, that was beneficial to the D community. The GDC backend has always been open, and for some time has been part of GCC. The LDC backend was developed to use LLVM. It's possible that there would not have been motivation for those projects if DMD's backend had been open from the start. DMD compiles fast but the performance is not competitive with the other compilers if you're working on something that needs to push the CPU to its limits.

I'm only able to work on the DMD backend now and then, and so it has fallen behind compilers with a small army working solely on a backend.

Sadly, the number of people nerdly enough to want to work on a code generator is very, very small. Me, I find it quite enjoyable.

I cannot even think of anyone who wrote a full stack compiler these days.