So, there are issues with something like inheriting comment threads in a segmented moderation space like Lemmy. Cross-posting a post from one community to another means crossing... let's call them "regulatory boundaries". Comments posted in Community 1, hosted on Website X may violate the rules of Community 2, hosted on Website Y. So, what would that mean in terms of rules enforcement?
As a moderator, you can delete the comments you've inherited, but it's a lot harder to keep up when you've just gotten 50 or 100 comments dumped on you all at once. It also breaks the syncronization you seem to be looking for.
You can decide that moderators at the receiving community can't moderate the discussion, but that's just ends up seeming somewhat parasitic, and a clear and open vector for abuse.
On top of the moderation issues, it also means giving users the ability to just... inject people into a community who aren't members, both without the consent of that community, and without the consent of the people being injected in. Like, what happens if I were to cross-post something into a troll community? Suddenly, I've just exposed dozens of people -- if not more -- to a harassment ring, with two clicks of a button.
Personally, I fail to see the upsides. This really just seems like yet another way to try and paper over the fact that we're all using different websites, and to ignore the websites that we're actually using in favour of make believing that we're in the centre of the panopticon.