Insightful, Google clearly doesn't care even though people would fall through the cracks no matter what they do. It's damned if you do, damned if you don't, esp. when you're a company the size of Google where people are just looking for a knee jerk reason to blame them for something. If they take a more restrictive approach with Android and ensure devices get app and OS updates in lockstep, the outcry will simply shift to complaining about slower updates, Google being too domineering, etc. One way or another, someone like yourself will say "Google just doesn't give a shit".
Talk about creating a scenario for people to claim, "Google just doesn't give a shit"...
First this is a logistical nightmare. There are dozens of handsets out there and they continue to proliferate. It's not "simple" to track updates on all these devices AND to also provide "engineering support" as you suggest. That's a significant undertaking.
Second, guess what? Even if Google were to undertake such a massive task, people would still fall through the cracks. What happens if a manufacturer/carrier falls short on their obligations - they aren't keeping a handset updated as proscribed. It's bound to happen at least a few times. What should Google do in that situation, revoke Market access for that handset? Would that be "giving a shit" or "just not giving a shit"? How would the owners of that handset feel, do you think?
Third, you're severely miscalculating the impact on Android landscape this would have. Carriers wouldn't submit to this kind of policy across the board, if at all, so they'd either drop Android altogether and switch to an alternative from competitors, or they would simply stick to the open source core of Android and fill in the blanks themselves. The carriers still hold most of the cards, not Google.
End result, a lot more than 5% drop in Android shipments and a truly fragmented environment that makes all current claims of fragmentation look utterly baseless in comparison.