I think mapgen has been great. There's been plenty of mapgen games going up, and it's not hard to fill one if you host yourself.
But honestly I think the main problem is there's just no real easy way to learn the game for brand new players. They basically get shoehorned into gap, astro, etc. Then they stay there because learning other maps is a big step. Setons for example, you need to learn a build for every spot and most good players just don't want to deal with having a noob on their team.
I think FAF could really benefit from new tutorial system that teaches general build orders that can be used on a bunch of maps, especially mapgen as well as getting into more advanced builds for popular maps like setons, twin rivers, open palms etc. As well as tutorials on specifics such as manual reclaiming, unit build orders in factories, scouting, eco management, things like that. Ideally this would be integrated into the FAF client for easy access.
Obviously that would be a ton of work, but I think it would be great for players trying to learn. As it is, it's basically try a new map, lose a bunch, get bitched out a ton for being bad, watch replays of better players and try to work out their builds for yourself.