It’s already been answered in the op:
Few examples of this:
- we don’t have to worry about versioning if we add something in core for a plugin, we know that both of them are at the same version
- easier to test a plugin depending on another plugin if the code is present for both
- when we change something in core we often have to do multiple PRs just to fix specs in plugins, now it means only one standalone PR
End result is more available time for the Discourse team to improve and maintain the product instead of dealing with this kind of issues, so eventually, you win too.