Good point :)
I'd hazard a guess that ActivityPub users are a small portion of the population, especially considering the people I'm thinking of setting a server up for are Facebook users in a medium sized town - mostly people over the age of 40 who only started using social media in the past ten years.
It's no secret that onboarding is a barrier to entry for Mastodon for example, and even then most people likely haven't even heard of it. I am the only person on any of my social circles who is aware of it.
I think it's fair to say that we can all end up regretting some of the stuff we've put online months or years after the fact.
For example, it means ActivityPub is definitely not an appealing place for young people to have their first cringe moments online.
The risk of being cancelled is higher, and I think everyone has the right to regret their past self and try to clean it up if they want to.
Update: So far many of the responses point out that if you post anywhere on the internet you are prone to this issue, i.e. even if you delete something someone may have copied it etc.
I do believe this is different. Yes, we are always at risk from malicious actors, but usually when using social media we can operate under the assumption that the delete button works. That is not the case for ActivityPub. Even without bad actors, your content may not be deleted. I think it's safe to say that this is an unappealing issue for most potential users.
When I post here I get replies. On mastodon I don’t.