I wouldn't say "never". I'd say decentralised social media grows more slowly but it's only a matter of time before threads does its own enshittification and there's another mass exodus to Mastodon. Sites that don't do enshittification because they aren't centralised and corporate won't have that kind of exodus, and will grow over time.

If they become so ubiquitous that they're the de facto standard, then cities will put their notices on them. You'll probably get official civic instances for notices, maybe hosted on their regular website domains.

I mean unless corporate social media finds some other way to subvert activitypub that's more effective than "look at me I have money for developers and advertising", then I don't see this trend changing. Corporate platforms don't seem capable of learning anything from their repeated failures, which is really strange. I think it happens because their hierarchies are inherently insulated against learning anything.