Low traffic → Less cars driving
How do you get less cars driving? People need realistic options. How do you get those options? Density to support transit and Density to support having businesses close to everyone's homes so they can just walk or bike. Car-dependent suburbia IS the problem, and the more of it you have in the city the bigger the problems of traffic and unaffordability get.
Finally though:
> want to keep them that way
Look, don't get carried away. Unless a very significant block of homeowners in your beautiful leafy neighborhood are itching to sell, you have literally nothing to fear. Maybe when a large mall is demolished a mile away from you, someone will build something other than single-family tract homes. Big whoop.
*Nobody is coming for your suburbs with bulldozers* just because "SFH" might not be enshrined as the only way anything can be developed.