I think MCP-UI in it's current state can fill that role very well, and that's not the only lens with which to view a SEP like this.
From what I can tell the main thing that this SEP does is put the official blessing of the MCP project on the existing underlying protocol that MCP-UI is already using. I think the better way would be to let MCP-UI and competing implementations cook for a little bit longer, rather than trying to get an officially sanctioned extension out the door and then iterate on that, which will cause a lot more churn.
For a non-trivial feature like this, I would expect an SEP that highlights more alternative usage scenarios and potentially operates on a more technology agonistic layer (e.g. it's very JS+iframe bound, so what about mobile or terminal UIs?), similar to the main MCP spec. Especially given the backdrop of the main MCP specification, which feels much more well-rounded and throughly considered (though in a few areas still incomplete), this SEP does not seem to meet the same bar.
Reading that from the perspective of someone that builds a LLM Chat UI[0] (and aims to be able to maintain that long-term) that heavily builds on MCP as an interoperability concept, reading what is proposed here and seeing how prescriptive it is in many aspects does not spark a lot of joy.
[0]: https://erato.chat
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EDIT: From what I can tell I'm replying to one of the co-creators of MCP. So let me just ask quite directly: Do you think that the risk of proprietary sprawl in terms of MCP-UI alternatives is currently greater than prematurely standardizing a potentially incomplete version?