I cannot see why they're discontinuing it. Anyone shooting with an APS-C body who wishes to use HSS will automatically opt for a new/used 580EX.
You answered your own confusion with the next statement. It gives people another option...which means Canon has to keep up the manufacturing line to produce it, stock the spare parts to repair and replace it, and provide tech support for it. MUCH easier to go with the new flash they just released.
That said, you'll see the 580ex in their refurb store for a while I'd imagine. So it's not like you can't get it; it's just that Canon won't cannibalize its own new item sales by offering too many options