Most U.S. cities cannot sustain purely-capitalist upzoning from low to medium; the profits for developers simply aren’t there, and the elected officials aren’t willing to upset homeowners. Three of the five cities I’ve lived in over the past fifteen years made the changes you describe to single-family zones. Developers largely did not, as you suggest, jump at the chance to build higher density housing. In one case where they did, their bank demanded they as part of the loan terms to set a minimum rent per apartment that was higher than neighboring single family homes, leading to persistently high vacancy rates, nonstop turnover, and a profitless development with 0% ground-level retail rented in the middle of a business-tourism district. So, for most U.S. cities, either they need to step in and commission redevelopment of the single-family homes of their voters, and take the profits or losses themselves on both their accounting books and their election outcomes, or upzoning simply won’t happen at all.