The earlier adopter of a forum with 70,000 posts are going to have the same issues. Perhaps give a user a general warning if their watched topic count goes over 50,000, and allow them to mass-unwatch topics with no activity in X months?
But really, there’s no “watching” of a category at all then, is there. It’s always, “watch for new activity” and the UI should be updated to show this fact. I’m rather embarrassed, actually, that I’ve been using discourse for years and had absolutely no idea that Watching a category was not going to apply itself retroactively.
This renders my idea here invalid:
https://meta.discourse.org/t/watching-a-category-perhaps-the-best-kind-of-mailing-list-mode/27507
Because, no, users will not get posts made in topics in which existed before they joined.