Android? Your cousin's Android may share nothing with yours, and so may your next Android phone with your current. You may not count on the same browser, assistant, mail, or maps being available in all Android devices.
You may argue that people will be sure to differentiate between Android A and Android B, but sadly, people have enough going on in their lives and want their phone to just work. Guess which OS provided the most consistent experience among devices and the most homogeneous app ecosystem?
From what I have learned with my interactions with non tech friends of mine, they don't want to know why such and such app is better than the default one. They just want things to look and feel the same and just work as long as possible.
You can argue that android already had custom skins on most brand's phones, but at least some platform apps used to be common. This may change even that.
There are deeper factors driving homogenization than Google Play Services. Those will probably remain, driving most Android users to a single fork. What this ruling does is make it possible that the next Android isn't dominated by Google.
Google could also pay them to install Google apps
I don't see how "unt[ying] the Play Store from Search and the Chrome browser," stopping "paying OEMs for portfolio-wide search exclusivity," or letting OEMs sell "devices with Android forks" helps Apple that much [1]. Android will still exist. Play Services and Store will remain Google's castles. OEMs will continue selling Google's bundle. All that changes is now they don't have to.
Worst case, Google adds a Play Services licensing fee. If this doesn't result in American antitrust action (for raising prices after forming a monopoly), all it will do is increase some phones' prices. If that matters to consumers, OEMs will be free to release non-Play Android phones at a lower price point.
[1] Stratchery