While that seems vaguely plausible on the surface, I honestly have to wonder if the vendors branch the sources because it is the most direct way to accomplish their goals.
It's possible. But looking at how the hardware OEM's operate (particularly at the level of the SoC vendors), the process from the outside looks a heck of a lot like "branch, patch, compile, rm -rf". And it's worth pointing out that the crap the OEM's mod into Android (Touchwiz, Sense, etc) plus the bloatware on top has been getting less invasive as time goes on and the vendors have been getting a bit quicker to pick up Android version changes. So there does appear to have been some improvement.
But at the core of it, "giving back to the community" and "smartphone OEM" aren't phrases that one typically expects to see together.
Let me put it another way: if Google isn't happy about this situation, why the fuck didn't they fix it a long time ago?
I think the carriers and OEM's are probably a lot less amenable to arm twisting than you think. The carriers basically lost complete control over the iPhone, so I can't see them being enthusiastic about Android also becoming a black box to them, and the OEMs are going to make what the carriers are willing to buy, plus they still want to have their crapware and whatever to set themselves apart from the rest of the pack.
It's worth pointing out that by now, the major OEMs probably have enough Android expertise that breaking off and building directly from AOSP is a feasible option if Google tries to flex too much muscle.
And if you think things are bad now, think of how much worse it will get if a substantial chunk of phones don't even have a common Google Play-based core capable of patching an ever-increasing set of components.
That's not even getting into the anti-trust concerns Google's going to run up against if they start adding more conditions to their contracts. They're already getting grief over "forcing" the bundling of their apps, imagine what they'll get if they start "forcing" their own updates to the core O/S (I'm sure the contract wouldn't be written quite that way, but we all know how it would be twisted).
At this point, the only proper "fix" I can see is for Google to keep doing what they're doing. Keep improving Android, building and improve their collection of must-have apps, try to maintain a market of unlocked Android Nexus/One/GPE phones, and keep some pressure on the OEMs to get with the program. I'm also quite interested in seeing how the Google wireless offering might go... if they create a carrier which only accepts unlocked phones and isn't trying to rape the consumer for profits, the North American carriers could be in for a well-deserved ass-kicking.