Especially since I believe grpc does take steps to mitigate some of the RPC issues you'd have with, say, Thrift. For example having the REST gateway so you can support the curl scenario without any extra work. (For example when building my own Thrift-type services I would do extra work to have a mini REST API for status and other operational curling uses)
As to the second part of your argument about there now being a second API, and how that is not necessarily a good thing, I quite agree and can't really speak to the etcd use case, other than that I don't think it's some property of 'Xooglers' to do such a thing to an open source component. I've spent my career using open source components: I've had lone developers who change their APIs willy nilly to fit the pet peeves of their own organization; I've had large companies own an open source component and have it be very disciplined about version migration; I've had lone developers be similarly disciplined; I've had companies make open source projects really complicated.