Bluesky-social-network, Bluesky's Relay will never not be the only one.
It depends on how you define Bluesky-the-social-network. If it's just in terms of Bluesky-the-AppView, then you may well be right. If it's defined as "the people currently using Bluesky", I think it's very likely we'll see alternate Relays and AppViews within the next year. Time will tell!
as in, this is significantly different from early Mastodon days when there was only one Mastodon instance.
Agreed. So we'll see how Bluesky and the ATmosphere evolve. My perspective is that Mastodon evolved in a way where Mastodon gGmbH had enough influence that aribtary decisions by Eugen led to innovation basically flatlining -- and Mastodon's dominance within the fediverse, combined with SWICG's inaction, has held everything back. So there's room for Bluesky and the ATmosphere to do better ... or to hit the wall in different ways. Once again, time will tell!