The point of mastodon is not that big companies will join small volunteer-run instances. They can run their own mastodon servers, just like they run their own mail servers and chat servers (or, more accurately, they outsource running their own mail and chat servers to Google and slack).
The right place for ActivityPub to land is that businesses and institutions will host their own (or pay a SaaS offering to do it for them, like they do with email and their CMS today), individuals will sign up to shared services (like you sign up with gmail for email, or blogger.com to blog).
The problem is going to be holding the barbarians at bay. All the same stuff we have to do for email will need to be deployed: Community blocklists for misbehaving servers, IP reputation, ML spam detection…
And funding it will require cash which the adtech industry will spiral in and offer to provide. ‘Federate with our servers and we’ll pay you 1c for every message you allow us to post to your users…’ Server admins will resist for a while but eventually, the walls will crack.
Enjoy it while it lasts, this burst of old school volunteer-run internet. It won’t survive this eternal September. It never does. As the architect of the Matrix said: Denial is the most predictable of all human responses. But, rest assured, this will be the sixth time we have destroyed it, and we have become exceedingly efficient at it