Running a mainline kernel on a cellphone

Google could indeed demand a specific kernel and it would gain a lot of traction like that, but it will weaken Google's position in the Android community. The more Google restricts it's partners then more expensive it becomes to partner with Google. The more expensive it is to follow Google's rules the more likely that somebody else to take over Google's 'benevolent rule' of Android (which would be much more 'friendly' in terms of proprietary driver encumbered kernels.). It's obviously contrary to Google's goals to make life more difficult for people that are willing to work with them then it needs to be.

The reality is that it's counter productive to try to compel people to act against their own best interests. You can't just 'force' people to be friendlier to the mainstream Linux kernel. How you win is by making it cheaper and easier to 'do the right thing' with the mainline kernel rather then chase down device-specific hacks/patches/kernels and closed drivers.

This has to come from a combination of improvements in the Linux kernel to make it more android friendly and participation of hardware chip set manufacturers to upstream their hardware support.