As I've said before, many businesses in the downtown Dallas area (including all the night spots in Deep Ellum) depend greatly on visitors who live well outside that area. Not everyone who works in all those tall office towers takes the DART train to work; plenty of them drive. Venues like Fair Park and the American Airlines Area draw huge numbers of people. I-35, I-30, I-45, The Woodall Rogers Freeway, North Central Expressway and Unsigned I-345 are all vital connections for moving those people to and from downtown. If the New Urbanists start removing major segments of the freeway network downtown it will kill business.
Traffic in downtown Dallas is bad enough as it is. Removing Unsigned I-345 would make matters even worse for motorists. Obviously that's part of the New Urbanist game plan, make driving so intolerable that it forces people onto mass transit. That strategy will not work. It will backfire.
Without a properly functional freeway network the people out in the suburbs will end up just avoiding downtown completely, spending all their time and money out there in the suburbs. There's lots of other places of interest in the Dallas Fort Worth area. I've made many road trips to DFW without coming near the downtown zone. As for the many thousands of people who work downtown, their situations may change if commuting to/from downtown becomes too much of a hassle. A lot of major companies have built large offices out in suburban campus style environments rather than the old convention of moving into a downtown office tower. Legacy Drive in Plano was an early example of this trend. Improving technology and Internet speeds are making "telecommuting" and home office setups more feasible.
In short: if the anti-freeway folks are able to start ripping out freeways in downtown Dallas they'll inflict a big economic downturn upon the downtown area.