But UI's have to be async if they want to be responsive, while the HTTP request-response cycle (which is what Node is predominantly used for) is completely sync

But UI's have to be async if they want to be responsive, while the HTTP request-response cycle (which is what Node is predominantly used for) is completely synchronous. Sure, things like WebSockets are changing this, but you don't really need an event loop if you have proper threads or a multi-process server (you know, like Node clustering, which you tend to need anyway once you outgrow the limits and pains of a single event loop).