A UI layer you can conceivably replace. App stores have incredible controlling power. But even worse is the Play Services platform: It means even if you get the APK on an AOSP device, it won't run because it's not built to run on Android, the open source project, it's built to run of Google Android, the closed source product.
The realization people need to realize is that Google effectively forked their own OS, and the original has basically no support.
My phone feels pretty functional without Play Services and runs all the apps I want to run.
Everything installed via F-Droid.
* Messenger/Chat - Telegram/Mattermost
* Browser - IceCat
* Mail - K9
* Twitter - Twidere
* Podcasts - AntennaPod
* Music - Apollo
* File Manager - Ghost Commander
* GitLab - LabCoat
* SSH - ConnectBot
* Contacts Sync - DAVdroid
* Maps - OSMAnd
Not sure what else I need to make my phone fully functional.
The problem is, people will often need one or two apps which open source can't adequately replicate. And they're all on proprietary Android only.
The Play Store, on the other hand, is taking a risk.
Also I think a lot of the movement of things into Play Services instead of the base OS is because manufacturers were so horrible about actually updating their devices so Google moved it out of the OS into a portion they could actually control so they could actually roll out updates in a timely manner.
Second, the OS, designed for openness, would have open protocols for apps to build on that could be backed by anyone's servers. For instance, you can add multiple location providers to Android, there's no excuse for telling developers to build a dependency specifically on "Google Location Services". An open platform would have the location provider in the open source OS, and then apps would pull from whatever location services happened to be on the device in question, whether that be Google's servers, Samsung's servers, or someone else's.
The reality is that Play Services was designed for one purpose, and one purpose only: To create a proprietary fork of the Android application platform that would only run on devices Google controls via their secretive contracts with manufacturers.
If Google really cared about OEM updates, it would make it as part of the requirements to license the Play Store.
As it already happened to a few that tried such attempts.
Amazon is probably the only one that can manage to push known app vendors to also publish on their store.
And using Play Services doesn't require to have an account with Google or to have a network connection. As Google Play Services are distributed for free, anyone can download them and set up so that they cannot connect to Google but still allow to run apps dependent on them.