I don't think it is a binary 'either its a new plane or its not' situation. I think every change has to be scrutinized for its safety implications, and for MCAS the analysis seems to have been, in part, that a failure could cause a trim runaway, but trim runaway is a circumstance for which there already exists a response, and that would not need changing (what this overlooked was that the pilots would experience it differently, as pulling back will not stop it, and if they stopped it with the trim button but didn't disable it, it would start up again ten seconds later.)

Putting a longer undercarriage on an airplane is not a trivial matter. You have to design it, test it, get it certified (a new undercarriage raises obvious safety issues) and put it into production. It's not just the undercarriage - it won't fit into the existing wheel wells, and furthermore, the legs might have to be moved further out on the wing, and then you have to rework that, as well.