Probably not google because google wouldn't play ball. I wonder if users will be able to (easily) sideload android apps since not everything is in Amazon's app store and apps there are often not updated as often as the google play store version.

Right, that was my assumption too. Sideloading would be my way around, but I'm guessing there may be some runtime signing or containerizing involved as well, else every Android vulnerability just became a Windows vulnerability.

I think amazon's appstore is free of google play services which could be a big reason

Not having google play services is a big reason the amazon store is a barren wasteland

Do you really think Google would allow that?

Honestly, yes. Someone else mentioned Google Play Services being a probable point of contention. Overall, with Surface Duo, Microsoft Launcher and Google's work towards making Flutter along with the leverage and developer interest they'd gain through having the Windows install base using the Play Store, it seems like a fair relationship.