A: Subscribr is really built for creating new content so for existing videos, it's not going to retroactively help with those.
Where it does help someone in that situation is what comes next: if the coach has 100+ videos, they clearly know their audience. Subscribr helps them take that knowledge and turn it into a faster, more systematic way to produce new videos that actually perform.
So less "fix the old stuff" and more "make the next 100 way better."