Well, sure - if someone wanted to be the hoster for all mail traffic on planet earth, then they would need a lot of resources and a business model to finance it

Well, sure - if someone wanted to be the hoster for all mail traffic on planet earth, then they would need a lot of resources and a business model to finance it. But if that is their plan, why would they have used ActivityPub in the first place? So I guess they could just shut their door as soon as the traffic is too much for them to handle. And tell people to look for another host or host themselfes.