Well this is the problem, USENET, like Activity pub was a PROTOCOL not a platform.
Eternal September lead the the collapse of USENET largely because no company could control it, thus it got harder and harder to manage / control in a way the normies wanted, and governments wanted.
USENET today is primary binary transfers, i.e the high seas of the internet.
It is not largely a communications system anymore.
We are already seeing that as people leaving Twitter, the nice left censored authoritarian bubble and find out that the wider internet is not left censored authoritarian so they are demanding every growing block lists of servers attempting to make a little walled garden for Activity Pub, which undermines the entire purpose of federated/ distributed protocols
"Eternal September" did not lead to the 'collapse' of USENET. It was largely replaced by web-based solutions in more user-friendly domains like AOL. And USENET eventually largely ended up controlled by a few large entities (ISPs) that had the resources to support the huge message volumes. Since there was largely no monetization mechanisms people would support for the needed resources, freely available NNTP servers quickly were dropped by ISPs. Also, it has to be said that the text-only nature of USENET was generally seen as old-fashioned and it was replaced by technologies that could support a better multimedia experience.
Fediverse has all of the same problems inherent in a distributed store and forward system and may suffer the same fate, but lack of centralized control is not the defining problem here.
One major way in which "this time is different" is that the financial cost of the hosting is a tiny fraction of what it once was, and doesn't drive consolidation so strongly towards larger-cap players. It's not quite to the point of "too cheap to meter", but definitely in the realm of where donations can work.
And the underlying software has had the time to work out a lot of the nuances of this design and how to get it to a larger scale successfully. It could still break if the whole planet took up ActivityPub, but it's a hell of a lot closer to the ideal than what happened with the pre-Web protocols.