SIP tried to do too much, was overly complex, XML-based and ended up doing nothing well enough to establish itself as a market/community favourite. "Nobody" outside telecoms does SIP, and I think that tells all one needs to know wrt market-penetration.
These days I'm rooting for matrix[1]. Matrix seems to solve the issues people want IM to solve, it federates, and supports bridging to other types of networks which aren't matrix-based.
It's not perfect. It may not be the final IM-solution to rule them all.
But for me at least, it's a good, working solution for a federated, decentralized platform (in the original sense, not crypto hog-wash sense) for the IM needs I have.
It might "just" be one step towards a better solution, but I'm sure the experiences people have with using and developing matrix is crucial in forming the solution-space in the right direction.
We're not going to get to a decentralized and "good enough" solution unless someone is willing to walk down this road and take the first bumps for the team.
I got my own instance, my own users, my own federated channels, serving my communities. Are you?