Yeah, this. If it were just about the OS customization Google could release a PC/Mac tool which would flash the latest build of Android onto any phone with an unlocked bootloader (which is most of them these days). The user could choose to continue with the phone manufacture's default build or get the latest "pure" Android.
Where this idea falls apart quickly is drivers for custom hardware. There's just no way Google could take on the driver development for every phone (both because it is just too much work and also because lots of phones have proprietary drivers running against hardware that isn't publicly documented).
Absolutely. From what I understand, one of the big reasons ICS updates have been so much more difficult than Gingerbread ones is that the driver interfaces changed for a lot more things in that jump (exacerbated by the tablet-only Honeycomb in between).
Where this idea falls apart quickly is drivers for custom hardware. There's just no way Google could take on the driver development for every phone (both because it is just too much work and also because lots of phones have proprietary drivers running against hardware that isn't publicly documented).