I can say with full confidence that I have absolutely no idea.
I’ve heard this idea thrown around before, so I take no credit for it: One way to circumvent the issue would be to have actual relay nodes. As in: nodes that don’t hold contents or users themselves, but just “broadcast” incoming messages to several instances, so that the source instances don’t have to. This would of course have its own drawbacks and limitations, but it would alleviate the bottleneck.
I’m sure some kind of solution will be found though. Call me optimistic, but I think the lemmi/binniverse has a bright future ahead of it. I, for one, have burned my reddit bridges.