That's actually true, because I'm always thinking from a cloud deployment perspective (which is my use case). What kind of architecture do you run this on, at scale on the cloud? You have very limited options if your monolith is on a serverless and is CPU/memory bound, too. So, that's where I was coming from.
The overhead of spawning a process is not the problem. The overhead if your runtime is huge and/or slow to start could be, in which case you simply wouldn't run it on a serverless system - which is ludicrously expensive at scale anyway (my dayjob is running a consultancy where the big money-earner is helping people cut their cloud costs, and moving them off serverless systems that are entirely wrong for them is often one of the big savings; it's cheap for tiny systems, but even then often the hassle isn't worth it)
I think you have an overcomplicated idea of what a "server" means here. For MCP that does not mean it needs to speak HTTP. It can be just a binary that reads from stdin and writes to stdout.