It’s not like operational costs are skyrocketing, or that hosting will become any more scarce or more difficult.

That remains to be seen. There are multiple ways a single huge instance could drive up costs for everyone else, especially when there isn't organic growth that allows developers to find creative workarounds to firehose problems.

Lemmy has been seeing federation-desync issues over the last couple of weeks due to a bug in kbin being amplified by Lemmy.world. I imaging a similar issue but with a fully federated Threads would simply ddos most fediverse instances out of existence.