> doesn't every recursive function have an iterative version
True in the abstract, yes. But it's a sophisticated compiler indeed that turns something like a recursive binary tree traversal into a loop (it would need to synthesize the stack worklist).
In practice, it's easy to do this for tail recursion (and mutual tail recursion, with a little more sophistication). You can get slightly fancier with "tail recursion modulo cons," which is a little more clever and handles map. Beyond that, it's pretty gnarly to do a good transformation, because recursive code is implicitly using the stack in interesting ways.
> couldn't... functional primitives be converted to imperative code
Indeed, and we do write those in pure JS with carefully-crafted while loops to make those primitives more efficient. But if students are learning to write their own map, or another functional combinator on lists, those need to work, too, and will be implemented recursively by them.
> Generator functions are no different than regular functions when it comes to stack depth.
Yeah, stack depth in general is annoyingly low on modern browsers, IMO, so this isn't just a problem with generators. It's also unpredictable (http://stackoverflow.com/a/28730491/2718315). So we're working around the normal stack limit already. I was sort of hoping that when a generator's continuation was captured, it would stay heap-allocated and not "count" towards stack space when restarted, but that's not the case.