Drop the scare quotes from "successful". These guys shipped products. The migration to multiple databases, syncing user information etc. is a milestone, not a necessity at every step.
As I see it the only way to look at scaling issues is that you’ve made it. Almost no user facing software will ever reach a point where something like Django couldn’t run it perfectly fine. It’s when you do, you build solutions, which sometimes means forking and beating the hell out of Django (Instagram), sometimes going into Java or whatever, spreading out to multiple data bases and so on. Of course once you actually hit the scaling issues you’ll also have enough engineers to actually do it.
You can't just keep "doing things that don't scale" forever. If you have 100+ engineers [1], you aren't a startup anymore no matter your roots.
[1] and remember, my comment that is the ultimate parent of this conversation is about not continuing to do this at 100+. I said nothing about scaling up to that point.