Amazon, Samsung, etc need to get to get together and build (an open-source) GMS alternative, to keep AOSP viable.
No chance. Amazon are a retailer, and that shows in the variability of their products. Samsung et al are manufacturers, and manufacturers do shit software. Nokia did well to keep it together as long as they did, but eventually it all went wrong. Look in other sectors (eg cars) and the onboard software is shocking - crippled, functionally inadequate, and performance and capabilitywise about ten years behind mobile phones, for no reason other than that car makers don't have a clue about software.
Even if they managed to bribe (say) Sailfish to conjure up an OS, customers would be back to square one, with all the device OEM's happily orphaning products within months of launch.