While increased control of the platform is one motivation, I don't think it's the main one.
Google's been forced to go in this direction by vendors who put out Android devices and never update them, even for the most serious issues. The only way to get security updates deployed is via Google Play.
Aye, I didn't say it was the sole reason and I agree.
Flagship devices with 18mths-24mths of support when Apple routinely does 5 years was damaging as well.
I typically buy mid/low-range android devices and AndroidOne has been great, I'm still getting monthly security updates on a must be 2 year old-ish Nokia 6.1 that is still a perfectly fine phone for my needs and it cost me 150 quid sim-unlocked.
Almost like that was the plan and Google employs some smart people isn't it.