Yeah, that would work within lemmy, and it would make it easier to detect whether a link is to lemmy or something else (look for /c/@/). But you'll still have the issue of clicking a link elsewhere (say, a blog post) to an instance that's not yours, so you still wouldn't be able to directly comment w/o copy/pasting part of the URL to your instance.

That said, that change alone would reduce a lot of friction for users. My point is that it still doesn't fix the root of the problem. I guess we could use a browser extension to auto-redirect to your instance of choice, but that's just yet another barrier for users.