I think the change happened shortly after Ruth Porat was brought on and the Alphabet reorganization was announced (Larry Page wanted to retire without having that be the headline). There was clearly a change in mandate to start monetizing more aggressively and you started seeing the ad load increase across all their properties. That all happened during a lull in Google's stock price. It started trending upwards after Porat signaled to investors that all the money pits would be cut back.
This ultimately affects Google's future growth and it's why you'll see Google do more such "desperate" moves like trying to become a military contractor, building its own iPhone-like phones, tracking users more aggressively, and I imagine android users will soon see os-wide ads, too.
Except they won't be OS-wide, they'll be Play Services wide and mandatory if you're an vendor including Play Services.
If you don't want Google's ads, you can go do your own thing on AOSP like Amazon and other largely-flopped attempts.
Over the last year or so, it feels more and more like Google, as a company, is getting desperate. Like it feels the external tide of popular opinion turning against it. But rather than mend its ways, a decision has been made somewhere to slurp up as much information as possible as quickly and quietly as possible, before it all comes tumbling down around it.
I wonder if GDPR was the turning point. Even my boss, who on a computer literacy scale of Linda Lovelace to Ada Lovelace ranks near the bottom of the scale, asked me to explain GDPR to her.