I don't think concerts are the main background work Holo girls have. Obviously for some people they are, but it's not for most people there's just gen anniversary and holofes.
It seems to mostly be merch prep and recordings for sponsors/conventions/special projects/group songs. I remember when the world of warships voice lines leaked, that collab was fully in the game files but for some reason didn't happen for a full year, I bet there are dozens of things like that which are recorded and are just vanished into the aether without ever seeing the light of day. Likely 50%++ of your oshi's work is going into a dustbin. That's the kind of work Hololive girls are escaping by going indie.