You can't definitively say that until it happens. It's also like saying when Pep does eventually leave the full squad of players will have to go with him because they're not capable of playing another system.
One of the things that has made Pep so successful at city and throughout his career is how he has constantly tweaked and developed how his teams play. The brand we play now is a million miles away from what made us successful.
What is stopping him from tweaking it to make us a little more difficult to score against? I'm not suggesting he overhaul the whole system, making us a little harder to score against would be a start.