This (asynchronous execution) means we are forced to use cooperative multitasking, which is probably not good for reduced latency, and therefore bad for responsiveness (compare to the Windows 3.11 days where we were forced to use cooperative multitasking).

Yes, I know most JS programs today use asynchronous execution successfully in practice, but to use it as the basis of a language platform is not a good idea IMHO. Especially when it comes to UIs, which need the responsiveness.

EDIT: web-workers are not a solution, because you can't share large data-structures efficiently between web-workers.