There is Samsung Galaxy store.There is Xiaomi store.There was Amazon app store which failed to gain traction as it was not pre-installed but had decent number of apps. This is even before we get to open stores like F-droid.
Now their viability is a question in the long term, if Google has complete discretion over what is allowed.
I also heard that dev's couldn't remove apps they, for whatever reason, no longer supported. Even if the developers abandoned Amazon's app store entirely, the old bitrotted apps would still be there. The 1-star reviews would pile up, biasing users against them on other platforms.
Google has essentially full control. Because they own security updates. Because banking apps already now often do not work without Google services. Etc. Etc.
It is a monopoly (regarding Android app developers), and is increasingly exploiting that to cash in.
They already have agreements for it anyway.
Is this wrong? Sure they have agreements but does it include "don't sell phones with alternative ROMs"? It's licensed software and I don't think the phone makers want to end up on the wrong side of a Google lawsuit.