AOSP is a compromise, because device manufacturers don't want to share anything. Google effectively negotiated with device manufacturers to open source part of their software. Device manufacturers lose some of their secret features to competitors. In exchange, they don't have to develop those features themselves. App developers get a standard platform, which benefits everyone: users and manufacturers and app developers.
This is very much a win-win situation, because the alternative is that every manufacturer has their own proprietary system.
No, it's a lose-lose situation. If we have 100 different mobile OS's it's a matter of time until a "good one" appears, and we get some sort of innovation in the space - be it from a technical perspective, from an UX perspective, or whatever.
Now we're all stuck with Android, where manufacturers can't really do anything interesting with their phones, users get an incredibly bloated, technically incompetent system, and all parties have to abide by Google's every whim.