We have data on what it costs to run a sizeable instance of Lemmy and it's not a lot. How does Piefed compare? Anyone starting an instance who envisions it growing large has to contend with this question. Currently it seems it's got a bit under 1000 users across under 10 servers.
There are now sizeable communities run on Lemmy instances that are reinforced by network effects. There needs to be a significant reason for them to migrate. To that point, the collective project is building communities away from corporate power, not software. The software is a tool to facilitate that. Lemmy has worked well so far in this regard. If someone can show that Piefed can work better and not cost significantly more, it'll probably get adopted for new communities. If the difference is drastic, we may even see migrations from Lemmy.