The push to lock down the Google app ecosystem is just Google's eternal quest to lower costs.
Reviewing apps, even if automated, is expensive. A while ago they demanded that all app publishers get a DUNS number. It's obvious why they did this - they want to rely on the reputation of the company (easy, automated) rather than detect malicious apps (slow, may require manual intervention).
One important thing to note is, the threat model changes depending on the user. If a "poweruser" installs a card game app, and it suddenly pops up a screen that looks like their bank login, they're going to close the app and report it. Grandma might get confused and think she was checking her bank balance. So they need a complex set of tiers and warnings to ensure that only users who know what they're doing get apps that could do something like that.
But, we all know that the above scenario is hardly the worst thing an app can do. Thanks to mysteriously bad security, there have been all kinds of Android vulnerabilities that allow one app to spy on another app's data files. Maybe locking down the ecosystem so that only "good" apps can spy on one another is the win/win solution for them.