This solution is incredibly stupid. This will kill the entire forum ecosystem

Hardly. There's obviously a lot of butt-hurt and other signature campaigns will get more competitive; as a result some of the people who are only here to shitpost for a campaign will leave. If anything this will lead to an overall increase in the quality of posts on the forum.
I rather all signatures be disabled globally, no links allowed, but never see a Bitcoin service banned. No banning of discussing Bitcoin services.
We can discuss gay porn, and porn in general, but we can't discuss a Bitcoin service? Welcome to new north korea.
He's not banning discussion of mixing services, only advertisement of them. I suspect you know this already but this is a pre-emptive measure to ensure that the forum itself doesn't get taken down. Theymos is simply trying to get one step ahead of the curve here.