With CGI the developer of the script could pretend that the-only-thing-which-existed was this request and do all kinds of things that would bring down a persistent process (leak memory, mutate globals, etc.) The problem was that spinning up a process per-request was expensive and slow. Now, with WASM's memory model it becomes possible to have a process that both does all the slow work initialization work once and has the ease-of-reasoning properties of CGI's "a single process for a single request" serving model.