It doesn't matter if he's not quite as good a shot stopper, his ability with the ball at his feet will allow us to play in a way which will save many more goals than we lose because he's not as good a shot stopper.
A Keeper like ter Stegen means it's easier to keep possession. Possession is the best way to keep clean sheets
A Keeper like ter Stegen = higher defensive line, which means more compressed pitch, which means if we do lose the ball, our outfield players are closer together, they can press more effectively, and because they are covering shorter distances they can press with more intensity for longer.
More numbers higher up the pitch also means we can stop counterattacks more easily, and a goalkeeper who's confident enough to run 30 yards out of his box to sweep up a hopeful long ball over the top allows us to get possession back in their half quicker which means the other team can't get out.
In attack, having an extra player actually capable of being in the passing triangles under pressure means we have a man advantage and that translates into one of our attackers being free and that leads to goals.
In short, ter Stegen would have to be absolutely miles worse than Hart at the traditional goalkeeping stuff for swapping the two to be make the team worse, because the way we would be able to play with ter Stegen in goal would save us loads of goals, and let us cut out opportunities before he even got involved.
And honestly the gap is not so great. In fact a lot of the errors ter Stegen makes could be attributed to his playing situation - he only plays every fortnight in the cup comps, which is no way to be as a keeper, anyone would make more mistakes in that situation.
If there is an argument for keeping Hart number 1 then it should not be made about his ability as a keeper, it should be to do with his position as a leader in the team and at the club, the fact he's a proper City player who loves the club and has been here a decade, which is the kind of player we need around and the fact he's English and pep is supposed to want an English core.
But even optimistic Hart fans must be hoping he can improve his ability on the ball over 6-12 months of training with Pep - anyone who thinks he's gonna be able to don't straight away is delusional ("he just needs more options!") or hasn't watched him for the last 6 years in our goal.
Meanwhile if we don't bring in another keeper, pep will have to put his plans on hold. Because we can't play the way Pep wants until we have a keeper who can sweep and is as comfortable with the ball as an outfield player.