This still exists... OsmAnd, offline map app for Android, has 10M+ downloads. Maps.me has 50M+ downloads. Sure, that's not 10B+ of Google Maps users, but still a lot of users.

I don't think the "advanced users" market has shrunk much, it's just the whole pie became so much bigger that the overall ratio decreased.

Isn't Organic Maps the open source successor of Maps.me?