Even if it doesn’t really matter which instance you begin with, the experience will be different, and there’s a sense of “pressure” at the point of signup, which doesn’t exist outside of the Fediverse.

Would you not say it's more like it doesn't exist to the same degree? Not that that diminishes your point, mind, only that in my experience online I've found similar when it comes to other online communities, say when deciding different Discord servers to join and some requiring waiting, reacting to be able to chat, or more rarely, have 2 factor authentication enabled of all things.

Before that, and more a sign of my age I guess, it would have been different forums, different chat rooms, and the like. Each similar in basic functionalities, but different experiences and a different sense of "pressure" to each.