There already are IM protocols. The problem is that companies prefer to create walled gardens. Even if a protocol were to take off, what usually happens is that one company leverages the protocol to gain traction, spends enough resources to build a really great user experience, and once they’ve amassed a big enough percentage of users, they toss the protocol and use something proprietary. You’d need the protocol to have at least two popular clients so that the user base is split between them. That might prevent one from dropping support for the protocol.