Less phone models and less customisation is the answer
A few years back, HTC were horrendously guilty of releasing new phone models every few weeks - they'd be slight variations on existing models and probably caused extreme customer confusion about which model to buy. It lead to an HTC culture of not providing many (if any) updates for a lot of their Android models, on the assumption that everyone would be getting a new HTC model once a year and nothing needed updating beyond that.
Eventually, HTC saw some sense and reduced the number of new models (I'd still argue there's too many!), but the "everyone updates annually" mentality is still there. It does surprise me that Google don't lay down some rules about this for everyone in the OHA:
* Minimum hardware specs. Nothing causes defections away from Android quicker than a lousy experience due to poor hardware. This issue is thankfully gradually fixing itself (e.g. Moto G for 100 quid at Tesco) - it would have been handier in the early Android years.
* Maximum elapsed period after Nexus devices get Android update for non-Nexus devices to update. If the manufacturer fails to provide a timely update, have a rolling points system where they can be thrown out of OHA if they exceed a certain total.
* Minimum update support period - Google has 2 years for Nexus devices, so why not other manufacturers? Again. failure to update for that minimum time would incur more penalty "licence points".
* Some sort of rules about the "bloatware" that manufacturers and carriers needlessly add. For example, nothing should be added that causes update testing to be extended beyond, say, a total of one month (again, with penalty points if this is exceeded). If this means dropping some of the bloat, than that's a good thing.