I believe that the issue has always been that a longer gear - absent telescoping features, which nobody really likes - means moving the gear attach points outwards, and a new center section. At the start of the 737-300 program, that was ruled out: the team had been told to find something to do with the 737, at a point when Seattle was doing the 757 and 767 in parallel. (That wasn't the original intent - the plan was for the 757 to be a mishmash of 727 parts, a new British-funded wing and the RB.211-535 engine.) Even so, widening the gear might also have meant moving the engines outboard, at which time you had a new airplane.