Very true. This is why I'm sticking with a server-heavy design (i.e. Rails) for my main projects. I agree with all of your arguments about the difficulties of testing. And I'd add that there's a distinct lack of tools for testing client-side. Sure, you can get unit tests for client-side JS, and you can run integration tests in Selenium etc.. But the client-side testing ecosystem is tiny and immature compared to that for server-side testing. This will all change, but for now, I don't envy anyone who has to test a heavily client-side app.