Do you believe Lemmy/Mastodon can become mainstream and fully replace their centralized counterparts?

I honestly think it's unlikely. Not because Lemmy is bad or that the tech couldn't handle it. But Lemmy isn't really profit driven - there's no way to really build a moat without defederating, and therefore no capitalist reason to advertise and grow a server - all that would do is increase infrastructure burden and then leave the server owner trying to figure out how to recoup the cost. And if they start running ads, charging fees or running people nuts with merchandizing, that growth they paid for is likely to scatter to other servers offering the same access to content.

So if growing tall isn't likely, what about growing wide? Well, maybe. I'm still extremely new to Lemmy World, but from what I can tell to run a Lemmy instance you have to have or be willing to learn a basic understanding of Linux, and be willing to charitably donate your hardware/bandwidth to the public. That might work out, or that might be constrained either by freeloaders scaling faster than donors, or the learning curve proving too much a barrier to entry. Wikipedia worked out, but it still has to occasionally prod its users to remind them it needs money to keep afloat.