Open protocols and APIs seem pretty meaningless to me if there's a single point of control for the brand.

You'd need to expand on this more for me to understand you. Yes there's a single point of control from a moderation standpoint (labeler), as there is on Lemmy instances. But anyone can host their own ATProto relays and the Bluesky relay will federate with each other automatically.

If everyone migrates to bluesky and then bluesky says "of we're not doing that open thing anymore because of this new embiggened thing we're doing" everyone will still be on bluesky.

Not necessarily because the accounts are atProto accounts and you can migrate to another platform(albeit another doesn't exist yet) without data loss. As far as the Bluesky app goes it really just shows you atProto posts and hosts your data (similar to Lemmy instances) they as an entity just also maintain the OSS backend Relay crawler and more.

I really think a lot of people have this perspective that it's not decentralized just because it truly is a lot more complicated due to there being like 5 different moving pieces of decentralization (PDS, Relay, Appview, tbd labeler, algorithm) and they do a great job at obscuring it for regular users which is a great thing. And nobody has really tinkered around and set-up any sites or integrations with it yet. I'm personally trying to get a two way mastodon integration as it's possible but nobody has done a solid implementation (just somewhat gnarly bridges between protocols)