How do we even know that they’re not already running instances? Why would they start announcing it, especially after the response?
For subscriptions, I would personally favor a “discovery” mode. When you sub c/abc on Instance 1, it tells you about c/abc on Instances 2 and 3 with the option to sub to them individually.
I like the idea of a single post that gets tagged to the communities it should appear in. Moderators would probably want the option to block or control something like that though, considering the risk of spam.
Gee! Sorry! It’s a pretty standard stunt, Homer.