I think I'm on Lemmy for the long haul - I like the fediverse decentralisation. The hardest part of Reddit to abandon will be the search results on Google, but perhaps we'll see something similar with Lemmy in a few years if it picks up steam.
I rally hope that Lemmy instances allow themselves to be indexed. Reddit had become a great source of information - I hope Lemmy instances can too. That information needs to be discoverable to be useful.