There is simply too much infrastructure to maintain, and more and more stuff is being built every day with no realistic plan for long-term maintenance. It's a losing battle.
Let the people who like dense towns live there, and stop trying to tell the rest of the people how to live.
Strong Towns also tries to creates cities where no one from the outside ever comes in, since there's no way of getting there (no parking, not enough roads).
People in large cities also like to "get away" during the summer - but there will be nowhere to go in his world.
Also, labor costs have gone up dramatically due to a variety of factors. Increased safety standards, less supply of labor relative to demand, increased liabilities due to tort laws, etc.
Also, borrowing from the future to build today is easy when population is exploding and you know everyone is having 3+ kids which will expand the tax base. That has not been true for quite a few decades now, with tax bases declining in many areas and political appetite for subsidizing those areas is waning, in my opinion.
Less future growth means more restrained borrowing today. There is also a significant and increasing amount of the country’s resources being spent on keeping old people alive that was not the case when all the old infrastructure was built.