As you said there is infinite content on the internet to import into Lemmy, no reason to steal the interactions from a small lemmy.world poster
From a very practical perspective, it's faster. The crosspost mechanic, that I indeed use, is just one click compared to having to rehost an image to an external hoster, copy paste that link to Lemmy, then give credits to the OC post (most of what I do on !lego@piefed.social )
So in this scenario the lw movie poster didn’t get enough engagement on their content and left… That’s a net negative for Lemmy
There aren't that many posters around. !movies@lemm.ee had a diverse mod team, several posters, that allowed to "spread the load". Potential new posters would see that and join the community with several posters instead of one single person. Having one community emerge as "the one" fosters more conversation happening there, compared to posts without engagements (which is a complain you raised in this thread)
Link aggregators aren't microblogging. It's expected that different people contribute to a community, if posters want complete control over their content then microblogging could be a better option.