Twirp is excellent. Their protocol is _very_ similar to the Connect protocol for unary RPCs.

However, the semantics that Twirp exposes are different from gRPC. That means that it's impossible (or at least _very_ awkward) to have the same server-side code transparently support Twirp and gRPC clients. The Twirp ecosystem is smaller, and has more variable quality, than the gRPC ecosystem; when you choose Twirp for your server, you're betting that you'll never want a client in a language where Twirp is painfully bad.

Our hope is that the Connect protocol appeals to the same folks who like Twirp, and that the server-side Connect implementations make it easier to interop with the gRPC ecosystem.