Possibly so for now. But do you think the providers are interested in making interoperability easy?

Even in the future if they are required by law to provide it, I wouldn't bet against the craftiness of the providers to invent some sort of network effect dark pattern to make it painful, if not outright impossible.

Just as an example, with Muse I can already see that the way they are thinking of making money is via taking a transaction cut, so it's not that hard to imagine that Meta can negotiate deals for txns that happens through Muse which won't be available elsewhere.

Obviously I don't expect them to intentionally help me move to the competition, but I do wonder whether obscure data formats as a moat are a thing of the past.

Your second point is where I'd imagine the future moats to live: Exclusivity deals with service providers. Things are already in motion with Amazon banning and Shopify explicitly inviting Muse; we'll probably see much more of that.