It's not just the Google Play Store. It's Google Play Services. They've moved all sorts of features out of the core AOSP libraries and into Services, which is a separate app that Google updates frequently, even on older phones where the manufacturer has otherwise abandoned its customers to an obsolete version of Android.

Because Services somewhat solves this obsolescence issue, app authors use it, and any app that depends on Services will only run on a phone with Services installed. And Google has rules and constraints, a whole contractual relationship thing.

I suspect those contracts are where the Android-related anti-trust action is going to go.