That would have been a critique of Thrift, and a valid one, but gRPC has an http gateway interface with semantics for how the RPC calls map to REST calls and JSON. As long as that is supported, then there does not need to be a regression.
But it's trivially true in the case where the provider does so wish (as it would be true of any protocol with a fully-functional HTTP gateway infront of it), and it's untrue in the case where the provider doesn't.
And the provider has to do extra work:
"[offering an HTTP gateway] required adding custom options to gRPC definitions in protobufs, and add an additional container running this reverse-proxy server." ( https://wecode.wepay.com/posts/migrating-apis-from-rest-to-g... )
This still seems like a massive regression in openness to me.