@lain you can look at it the other way around too: what arguments for/against activitypub?

+: it's much easier to implement and maintain, it adds a few properly specified things
-: muh gargamel controls the federation

best outcome: both gs and masto support AP or have gateways, with many other great stuff compatible;
worst outcome: gs finally gets rid of mastodon and we live happily ignoring each other