> I'm serious here, if you're serving thousands of clients in a web app, why wouldn't you want parallelism?

For the same reason you don't do parallel by default for every loop you write in java. It's overkill. You can successfully argue that multiple service calls across a network need to be parallel, but this is relatively little to do with a desire to serve many clients and a lot more to do with responsiveness. (a noble goal)

> but I love the ability to iterate rapidly in the repl on a web app without having to restart my JVM

HTML and mixed services > (any combination of java technologies you can dream up to serve web apps)