I never thought adding a paywall was necessary. I was more thinking along the lines of a game being made easier to mod and its developers embracing the idea of modding like you kind of mentioned. This could be done by releasing tools to make it easier like Cities: Skylines 2's recently released editor and Hatred adding Workshop support. I don't know if official mod support Java would entail something like a built in mod manager, updates to improve modding capabilities, or some kind of universal package for mod files.

This doesn't need to be done through a service that the developer has any control over. SimCity 2000 had a Build Architect Tool players could use in the mid 90s and sites like ModDB and the Nexus exist.

I’m not sure what the latter is called, but I’m quite sure there’d be a massive uproar if MS/MJ did that for java edition. I know I’d never play the game again, that’s for sure.

There will always be the option of raw dogging files into the game directory or developing external tools like people did with Mass Effect.