The free market will sort it out in short order.
We're self-sustaining primarily through property taxes and have been that way for more than a century.
From the tone of other commenters here, I sense a malicious undercurrent intent on extinguishing rural ways of life - any excuse will suffice.
Like how the residents of New York City are able to get the food and water it needs all produced within the five boroughs? Or is that a different kind of "subsidy" from the hinterlands.
It's nonsensical to say that just because the tax receipts of a suburb doesn't pay in full its roads and bridges, its existence is necessarily subsidized by the rest. Does a city pay in full for its share of the US Navy whose protection of global commerce it depends on to survive? Are cities subsidized by the blood of those in the armed forces, who primarily come from the hinterlands?
Yet.
"In every region of the country, recruitment rates were higher in rural and exurban counties than in urban counties. Only urban counties in the South had recruitment rates above the national average. Cities in the Northeast, Midwest and West all had rates well below the national average." [1]
[1] https://dailyyonder.com/largest-share-army-recruits-come-rur...
Or were you planning on creating isolated servants in rural area who only work all day and never go anywhere?
You missed a very good option in between: incrementally densify them, which also has the effect of increasing the tax revenue per unit of infrastructure.
Of course new suburbs rarely are built with transit in mind while the oldest were built around the long gone streetcars. This build form doesn't have any affect on solvency though.
It's the opposite in my 1920s streetcar neighborhood (alas without streetcars today). The average lot size is 1/10 acre. That's enough for some small scale gardening (tomatoes, potatoes, broccoli), but nothing that is going to produce a significant amount of food. Walking distance grocery stores (and even street produce vendors) were a big thing in those days. Cars were shoehorned in during the 1950s.
Our lots are tiny compared to the typical suburban California lot, which is more like 1/4 acre, and dwarfed by typical American suburban lot sizes.
I live in a suburban area that has been around since the early 1900s and we're not suffering infrastructure collapse. Your number smells like BS.
Right now you're forcing me to pay for your fetishes.
Popular claim made without sourcing any facts.
But the discussion was about suburbs. The number of people living in rural areas is by definition low enough to not be a meaningful concern.
Infrastructure only lasts 30 years. We should be sunsetting suburbs, not expanding them.