Any instance hitting one million is unlikely, on the mere grounds of trying to make one super instance is kind of the opposite of the goal of federation. The winning would be reaching a million members between all instances.

To a "normal" user, a Lemmy (or any federated) instance is just another Reddit-like site.
If a user signs up and see content in their feed, why do they need to care about federation?

The federated system gives "normal" users the content they want, and "technical" users the ability to self-host and connect to other federated servers.

I think a handful of popular federated instances will see the majority of Reddit emigrants who don't need/care to know about how federation works.