What if the public transport solution involved policies like taxing the existing suburbs in a way that ensures a break-even on infrastructure upkeep and operating costs so that the people who live in them aren't unfairly pushing costs onto other people in society?
We could go a step further and tax automobiles in such a way that they covers the negative externalizes that their use results in so that again, the users of them don't unfairly burden others with the cost of their choices?
The money generated from these endeavours could be used to fund the development of mass public transportation infrastructure in more, dense, sustainable areas as well as things like high speed rail to connect those regions allowing the people who chose to live in those areas an easier and cheaper day to day life as well as the access to distant cities and all the economic opportunity that this encourages both in the destinations and in the economic corridors that arise along them?
Places like the US and Canada may be able to stave it off a bit longer because of relatively open immigration policies but the demographic collapse is coming. That means that cities are going to start fighting to attract a dwindling pool of people and cities with less debt, be it financial or infrastructure will fare better than those that have suffered under decades of perpetual growth mindset based policy making.
Many suburbs are a rot on a city's finances and will inevitably need to be amputated. The sooner the better I say.