I can't really think of a technology less subject to lock-in than REST; reliance on specified resource representations and HATEOAS makes it agnostic to pretty much everything.
EDIT: I guess there's lock-in in the sense that once you have REST, everything else is too limiting.
> Once we evolve past REST
Few things even reach REST, much less evolve past it.
> instead of being able to decouple cleanly from the underlying protocol
REST starts out decoupled from the underlying protocol(s), it's not something you need to do when evolve past REST.
> Everything should have been focused on the payloads, not the transport layer.
The only thing that REST says about the transport layer is that one should avoid altering or extending the semantics of the underlying transport protocol(s) except as necessary to cover gaps that have no suitable coverage. REST is all about the payloads plus HATEOAS.
Edit: for those interested, here's HTTP decision diagram that REST takes full advantage of: https://github.com/for-GET/http-decision-diagram/tree/master...
S3 API for example is probably a counter example. But it shows that the lock in is not inherent to REST, no?