Let me take the other position in this comment: I also see that how MCP works really helped its quick adoption. Because you could just build a local MCP server as a proxy around existing APIs and functionality, there is no need to touch anything existing. And MCP often starts as "MCP Server" that is basically a software artifact that you'd just configure and run - often locally. I don't think that just doing REST or extending existing REST APIs wouldn't have delivered this part of the MCP success story.

But now that many companies focus on MCP as a remote API, the question obviously comes up why not just use standard API protocols for that and just optimize the metadata for AI consumption.