yeah there are many options but if 99.99% uses gms on android. Its majority rule as a app developer i would push other services way back down in priority.
It's not about what is possible, it's about what developers eventually pick. While some privacy-focused companies seem to use more open alternatives or even implement the system themselves, most of developers simply stick with Play Services, which makes use of many (even FOSS) apps cumbersome without them.