Also the biggest difference is, that soap had like a trillion implementations which all worked kinda differently. code generation, etc.. GRPC somewhat does not have this problem because basically there is only one client implementation managed by google (now the cnf).

also in soap you basically built your server first, because writing a wsdl from scratch is like... akward. the idl of grpc is extremly simple to actually start without any implementation at all. and as a bonus it works way better if you need to add/change fields.