You are three years off....
Sakura gakuin typically recruits 10-12 year old kids. This is the group that spawned Babymetal. God knows how many similar group exists.
On the topic of haachaama. I actually think even if she didnt join hololive she'd still be big and will inspire just as many. She basically had 0 support from hololive due to her situation. Holo really did not contribute much to her success aside from her model and maybe a phone.
The one difference is she'd probs have quitted after that 8 months health break.