The game is so insane popular the modders will adapt or new ones with appear even if it's a huge pain in the ass. Since they still support installing old versions of the game it's not so dire.

If they have to put in all this effort to port their mods to Microsoft's new code, what about also porting them to an open source alternative

Might make some mods that support multiple versions stop at 1.20.1 but any new ones will be fine on the new ones, various API, helper mods, and surely some OpenGL->Vulkan translation ones will come in as needed.