I think there’s a legit use case for this if it’s done carefully. A lot of people aren’t on Reddit because they want to be, they’re there because their communities are. When those communities are small or niche, there’s often nothing comparable elsewhere (yet!).

A mirror community can act as a bridge. It gives people a way to start using Lemmy or PieFed without feeling like they’re completely missing out. Migration isn’t instant, it’s gradual, and without some level of content to start with, people won’t stick around long enough to contribute.

I agree that simply blasting communities with everything from Reddit with no limits is the wrong approach. But I do feel that a more targeted, slower mirror for a specific community can help people transition over time and eventually build something independent.