AP server admins will likely de-federate the largest ActivityPub servers, like Threads and possibly Mastodon.Social if they start generating a lot of inbound traffic on their instances. I predict the AP network will split into the network run by commercial/big entities and the network run by small hobbyists/idealists. Running an AP server that scales to the traffic generated by Threads will be no easy job and not cheap.

Running a Personal Data Service (PDS) on the ATProto network should be much more doable for hobbyists. The relay and app view servers handle search, discovery, and caching to alleviate traffic. So a typical PDS should not see too much traffic. Now just waiting for the Bluesky ATProto network to actually go federated, but I don't think we are too far off from that happening as they've already federated their own PDS into multiple PDSs.