This is a very good subject, but unfortunately it leads the conversation in a perpetual circle.
Android, being open license/source has made Android what it is today. It is the very idea that anybody can develop and push OS features in their own version, overlay, or for someone elses version. It PUSHES the OS to grow and have features. But Google is not a OEM Hardware Manufacturer, so they can't know how or what has been exactly changed in the OS to make a specific device work correctly. And this is the impossible part for Google. If you want to know how difficult it is, just go to XDA forums and read just one Custom ROM thread about the trials and failures on just one device, let alone hundreds.
I do agree they need more control, especially for security updates. OEM's are NOT software developers, they build things. They want to keep the carriers happy by building in carrier services and bloat into the OS. And they won't put enough money behind software development to keep updates coming out as they should. OEM/Carrier versions are slow, ugly, riddled with security holes, and lose support way to fast. We deserve better. Google is powerless to stop them being that the OS is free and OEM's just don't care enough beyond selling a device, while the carriers try to wash their hands of it and blame Google and just want bloat to tie users to their network.
So it just goes in circles. BUT, there is a solution in my opinion. Android is big enough now controlling most of the smart phone market. That means Google wields a lot of influence. But with so much money at stake for everyone, they are moving slow. The answer is unlocked devices taking the carrier out of the picture. The more unlocked devices the better. This is a trend that Google is pushing and they need to push harder.
Carriers need to be out of the picture, then the manufactures can feel free to give the buyers what they want without influence and directly update the phones. Carriers are fighting hard against this change and are putting enormous financial pressure on OEM's to not do it. A lot of people still buy their phones from carriers and the manufacturers are afraid. Google could speed up this change and they should. Maybe special Google apps with more free features or extra cloud storage for vanilla Android unlocked devices or incentives to unlocked device manufacturers (they got the money), such as paying the licensing fee for better camera software/drivers that keep many unlocked phones away from the best camera category because of price. Or offer Google financing to unlocked stock android devices to pull people away from the carriers. There is a lot they can do, but haven't.