But the Runaways did replace members.
https://marvel.fandom.com/wiki/Gib_(Earth-616)
So let's use your logic what makes something good, focus on the main cast. Billy and Teddy are busy, and America at the time also has a book (she is also a new person so it might be better). Which means we have Kate, Eli, Tommy, Cassie, Jonas (can be rebuilt). Basically the OG team minus 2. If that is the case adding members to fill those 2 slots wouldn't push anyone away because they are busy doing their own thing right?