"But the real problem is a total lack of GMS alternatives that app developers can reliably target"
Only one problem. A lot of those GMS services are real utilities: as in you need a lot of infrastructure investment to really do it right. Google actually plunked out of its own pocket to make its own maps; the only open alternative is OSM, and that relies on user input that can be hit or miss. What about app stores, security, and all that middleman stuff that Google can all bring together because it's all under one roof?