Even just using IAM to grant permissions is going across those services is going to lock you in.

The current trope I see going around is that lock in centers (almost) 100% around identity management and access control, if you control that, you control your customer’s destiny. You can move a key value store anywhere on a whim.

Sure, but there isn't a gun to your head. Use mysql or postgres on each provider if you're worried about that.