If the operation is expensive, that doesn't work. Say, if the user clicks a link to view a 2GB file, then changes their mind to view a different file instead, you really don't want the browser to continue downloading that 2GB file only to discard the result afterwards.

But the problem is that pretty much any operation is potentially expensive. Sending a REST request might become expensive when your phone has poor signal. All of the sudden all those "inexpensive" REST calls you just ignored are queuing up only to have their responses discarded when they arrive several seconds later, wasting bandwidth when you need it most.

If the language doesn't make cancelling easy, developers won't support cancelling -- and then you end up with unresponsive apps.

Yeah, cancellation in most systems are a "patch" (as you said, you don't know how long it will take to propagate (again, assuming distributed system, but this is less relevant when we talk about multicore system, with multicore system, that really is just about when a boolean propagated to all cores from false to true) but a necessary one and need to be integrated deep in the runtime. C# as an example, passed cancellation context (I believe in C# they called cancellation token) through all their async API as optional parameter. Maybe this is something Swift should take some inspirations from and will be beneficial for client-side developers.