I'd love to see MagPul make P320 mags, too. Although I can understand why it would take longer for them to do it since most of their established production capacity is in polymer and you really can't make a polymer P320 mag work unless it's a single stack.
The opening in the FCU of the P320 is very narrow, and being that polymer has to be molded a lot thicker than metal, the only way you could make a polymer mag that would fit through the FCU far enough would be for it to be single stack. Which, frankly, I wouldn't mind all that much since I'd keep my original metal mags loaded with Hornady hallow points and then use the polymer mags on the range, since my local range has a dumb rule that you can't have more than 6 rounds in your mag anyway.
But anyway, yeah. The fact that the P320 has been selling like hotcakes for so long and nobody has stepped in to make a killing off the demand for inexpensive aftermarket mags yet is shocking.