I believe the only reason they didn't do this out of the gate is because device manufacturers wouldn't have signed on for it. Android is establish enough at this point that it's not like many device manufacturers could really step back from it at this point.
Treble is a strong step in this direction - but they need to take it farther, open up customization APIs and drop the garbage manufacturers pump into devices. The way Treble is right now it only benefits power users who have their devices rooted - who could update without the manufacturers permission anyways.
I wonder if their only hesitation at this point is the mess they're making with ChromeOS and all their other garbage... I think the Android roof was the right one to bet on, massive numbers of users with primary devices running it, but I fear they've done significant damage with their other offerings.