It would cause traffic change, sure. But that change could be modeled, tested, and adapted around beforehand. Hell you can even go a step further and implement more routes for the free mall ride, move parking and traffic even further from the city center and use it as an opportunity to promote public transit. Sure nimbys would complain, cause that's what they always do, but when they see how much easier it is travel to and within downtown, how much quieter it is there, and how great the now enlarged park is surrounded by stuff to actually see and do they'll pretend like it's always been that way or that they were always for it.