Hard to blame them. Proton is dang impressive, and if it works, it works.

This isn't made by valve. As I understand it it's very difficult to make a source game for Linux because the version of the engine that these devs can use is like, 10-15 years old.

i'm happy as long as the game works properly

Isn't it kinda crazy that Proton is used not only as a way to play Windows games without Linux support, but also as a specific target because it's more stable than Linux? (Stable as in "works predictably like expected", not as in "doesn't have bugs")

Linux as a target is super difficult due different kernels, drivers, distros, DEs, libraries, services and so on. Proton just abstracts all that away and thus is a much easier target to develop for.

I wonder if at some point Linux will just implement the relevant API calls and such natively, as popular demand and usage grows

I did not enjoy the mod. Perhaps I'll try it again someday.