I'm not sure who is pushing #GlobalSwitchDay probably won't amount to anything, some of these platforms are certainly not ready a massive migration. The #RedditMigration was rough enough on Lemmy, it actually murdered #Kbin; #loops #PeerTube and #Friendica don't have nearly enough well-maintained instances to support any sort of massive migration. And Signal, well its better than WhatsApp, and its mostly Free Software, but it doesn't really align with all of the others that are, or at least intend to be in the case of Loops, federated services based on ActivityPub.
I agree it woudl be a big ask even if the platforms could support it. I don't precieve anytime that it would even be a good idea to directly encourage such behavior. The #twittermigration and #redditmigration where natural occurences that hit the 'verse hard. I believe they were a net good for ActivityPub and Free Software as a whole, but for many users the hiccups at that time where enough to sour them to "decentralization."