That growth will be mostly on android, but there's plenty of opportunity for it to be on windows phone as well. Microsoft has made some smart moves here. They've used windows 8 as a foot in the door to get people used to metro. The pc market may be shrinking, but there are still a billion pc's out there which will gradually be replaced with windows 8. They've also made sure that windows phone runs well on low budget devices (better than android even). And they've made sure that developing apps for windows phone is easy and cheap.
At this point i would not count microsoft out yet. Android is going to be the dominant player, but google seems not to care as much about the app ecosystem as microsoft. If google doesn't fix the piracy issues it's not inconceivable that windows phone becomes a more attractive platform for app developers. I hope this will be a three-horse race, as neither android nor iOS are a great fit for me.