MCP vs CLI is the same discussion as between a GUI app and a web app: it's all about the distribution. There is approximately no difference in functionality except whether you're hitting a dedicated service or running a local tool which connects to a dedicated service.

With saas is turned out that distribution to a browser solves a pretty major pain point and I expect MCPs to be treated the same. Can you trivially replace an MCP server with a CLI tool which accepts a token? Yes - but why do that to yourself when you can hit the endpoint directly?