This varies a lot across the country. Most of the local suburban streets are maintained through a combination of local city/county and gas taxes and don't come from the "city money" general tax pool. Not to mention that most states don't have a dense city to speak of, so they are in practice >95% low density.

The federal government maintains the highway infrastructure (and subsidizes other non-highway building) which does help enable suburban living in many places. I suppose that you aren't advocating to get rid of that because the immense usefulness of such system (travel, trucks, buses etc), though.

Eliminating the suburban lifestyle wouldn't have a massive impact on sustainability of the system because a lot of it isn't maintained by the federal government or simply needs to be maintained anyway. Reducing the number of cars that are on the road due to the suburban lifestyle will reduce the amount of money that comes from the federal government and shared money pools, but probably not too much in the grand scheme of things.

Thus, I think that saying that "suburban living is fiscally unsustainable" is an exaggeration and we should instead say "we need to better allocate the costs of suburban life to suburban taxpayers" (which wouldn't make such a huge difference anyway, especially if we also remove the city living subsidies that come from rich suburbs).