That may or may not be true about Manning being on this team, but you're comparing a quarterback that can drop back make his reads and throw the football with confidence that his O-line will provide the necessary protection for him. Ben, on the other hand, has no such confidence (at least it appears that way) and, in reality, Ben's protection is inconsistent which may be worse than always bad. Ben never knows if he'll get the time or not, Peyton is almost certain he will get the time.
Difficult to assess how a different quarterback would operate with this O-line.
Manning can make up for the deficiencies of the offensive line. he gets rid of the ball quick and is smart enough to not throw it to the other team.