Well duplicate communities can thrive both on reddit and Lemmy. You can imagine a community with several thousand users on Lemmy would still be relatively active enough to have quality content and discussions. In fact I've noticed this myself and it's only getting better over the past few weeks.

Rather than all of nothing approach, just think of it as both can co-exist for now. Eventually let's hope reddit will die its slow death it won't be anything like the death spiral of Twitter.