Jim Platt had already been at the Club for a couple of seasons before Jack's arrival as back up to Willie Whigham as had Stuart Boam and John Craggs. Graeme Souness had arrived around a year before Jack Charlton. I'm pretty sure Jack Charlton's only tweak to the existing squad was to bring Booby Murdoch in from Celtic. That's what made his success all the more remarkable.
Fully understand what you are saying. But there are loads of factors to take into account.
Fitness will still play a part
Any knocks/injuries too
Then any tactical implications that NW is trying to mitigate
But I do agree with your last point. Anyone not showing the correct attitude needs shifting out