You have a really good point & it's something I was considering while making my original post but couldn't address it concisely so I left it out. So here's my non-concise ideas on that.
My general take is that WP & the Amazon phones seem to be something more akin to the iPhone: an attempt to make an integrated hardware/software device (or at least one built to strict standards) that has a real distinct brand & tie in to the related manufacturers universes (or more cynically, walled gardens). Andriod, for better or for worse, seems to be a lot more decentralized which probably appeals to the device makers like Samsung whose main interest is moving handsets, not creating a "wonderful" experience for the end user.
So my comparison with Apple, MS, and Amazon was assessing the battle for their respective Universes & Apples dominance in that regard.
However...I see in hindsight that Google _absolutely_ has its own Universe that Android most certainly is a big and successful part of it: Search, Mail, Mobile Devices & Personal data / advertising. It's a mutually beneficial relationship w/ the handset makers because Google wouldn't care about hardware revenue and Samsung etc. don't really have a stake in those other "interests" of Google. It seems obvious that this partnership is a productive one for both sides & would be successful. I guess I just didn't notice it at first because of the divorce from hardware.
Maybe this is exactly why MS bought the handset division of Nokia. Playing the decentralized, hardware agnostic, Google-style game hadn't panned out, so making their own handsets is a further step in creating a competitive platform to Apple's. It may well work out for them over time.
That leaves Amazon's phone, essentially a branded/crippled Andriod, worst of both worlds and, to me, likeliest to fail due to lack of purpose.