It's not just the lack of apps (but any Windows Phone user will tell you that it's a huge drag) but that the ROI of offering an Android app outside of Google Play isn't there.

If it weren't just Amazon's AOSP, but also (for instance) if Microsoft and Firefox also offered their own, and combined they took a respectable share of the Android install base AND their respective app stores could all be targeted by Android devs with a single deployment then we would have something.