> you’re vastly over estimating the overhead of processes and number of simultaneous web connections.

It's less the actual overhead of the process but the savings you get from sharing. You can reuse database connections, have in-memory caches, in-memory rate limits and various other things. You can use shared memory which is very difficult to manage or an additional common process, but either way you are effectively back to square one with regards to shared state that can be corrupted.