The reason I'm asking those questions is that a customer of mine has a service with a JSON API, a Vue frontend and a score of customers using that JSON API. We know that the newest ones are using bots to code their clients (and they are using them wrong, by the mistakes they do.) I don't see a near future in which all those third party apps become LLMs that would benefit from a MCP server and we retire the API.
But yes I agree with your point: for a simpler app with a more traditional web UI it's likely the API used by the front-end would largely overlap with the user-oriented API. Then indeed the REST API has to be maintained for as long as humans continue to use the front-end.