IIRC, right before I left Google at the end of 2019 you could be added to an allowlist to use gRPC internally. I believe they want to eventually replace Stubby with gRPC. Someone who still works at Google would have to comment on the current state of the world or if my memory is failing.

As for Stubby it is a completely different codebase and is tightly integrated with a lot of internal tooling and features.

One other thing I miss from Stubby C++ compared to gRPC is that async stubby is super opinionated about it's threading model and came with batteries included, where I find the completion queue approach very hard to use.

Lastly, as far as load balancing goes. I think some load balancers with native gRPC support allows you to let different servers get different individual requests instead of requiring all requests go to the same server. Of course this only really works for unary RPC calls. But load balancing HTTP/2 isn't unique to gRPC nor keep alive http/1.1 connections. (Outside of the streaming RPCs of course, but there is an equivalent of websockets)