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Hololive members that actually want to be idols/have music careers won't leave. If they leave they have to abandon everything they've built in Hololive, their songs, their music videos, their recognizable name and branding, hundreds of hours of effort and even money they've sunk into their careers. They'd have to start over pretty much from zero and would need to find another agency to get them gigs and promotions anyway. If they gotta have an agency it might as well be Cover.
Very different situation from members who only care about streaming, since "starting over" for these members means simply getting a new model. They can literally quit and start streaming as indies the very next day, taking most of their audience with them. The only thing they have to relinquish is being able to collab with Hololive members but this can be largely mitigated by snuggling up to Vshojo members or other large indies that they're probably already friends with.