In what universe is anyone proposing tearing down the buildings in NY, London, Tokyo, or Shanghai and replacing them with SFH? I've seen some articles about new mega-rich construction in London with homes going deep underground, but largely replacing existing townhomes and the like, not lowering density. There are just very few people who would spend enough money to buy up a bunch of units and turn them into a handful of houses...
The only push for active change is the opposite direction: people who want to change the cities with abundant single family homes.
As a result there is less of a drive for people who are in between, or even want to live in the city a bit, to live there and make it a more lively area since it ends up being artificially more expensive.
On top of this, a lot of zoning makes interesting, dense cities straight up illegal or incredibly expensive to build, again, in preference of low density single family homes.
This is what people are concerned with and want to change.
The city also has the highest wage tax in the nation >4%, more than half of which is still due if you commute in. Unsurprisingly, these suburban commuters represent a huge line item in the city's revenue - that is gone if everyone is WFH. City schools also receive the bulk of their funding from the state, unlike suburban schools which are paid mostly out of local taxes.
In the Philly area at least, the city would never survive without its rich suburbs.
Maybe not "torn down", there was definitely a period of time in the US where urban areas were being abandoned for the suburbs.
COVID-19 will kill megacities. Whether they'll all be torn down and replaced with single-family dwellings or just left to rot into slums is indeterminate, but they're going to go away one way or the other.
Cities over a certain size are only viable if they have an effective public transport network, and public transit isn't compatible with endemic COVID-19. Get one superspreader on one train car and you now have 250 infected people and multiple deaths. Even the vaccines that are currently under development would only reduce the carnage to 125, rather than 250, people. No one is going to accept this risk if they have a choice, which is why public transit ridership has cratered everywhere.
Remove public transit from the picture and you have a car dependent city. By definition car dependent cities must have a low density to keep traffic congestion barely manageable. Highrise multifamily is no longer possible, nor are high rise offices. SFH with suburban office parks are the only style of building that makes sense when everyone must drive.
Further, everyone who can is shifting to WFH, which will erode the importance of cities as a concept. Get good Internet into the rural areas and most white collar workers will live there just to reduce the risk of dying from COVID.
When, in a few decades, we eventually get COVID-19 vaccines and treatments good enough to go back to the way things were, the changes to how we live will be socially baked-in and it will be too late to walk them back.
Cities are over.
I love to live in a city. I love to have access to museums, gardens, universities and good public transportation.
My fear is that people may end up killing the city in favor of single-family homes and making owning a car mandatory. That cities that I love like New York, London, Tokyo or Shanghai may disappear and become an endless shapeless blob of single-family homes without identity were people looks suspiciously to anyone that "do not belong to the neighborhood". It would be a loss to humanity to get rid of the creativity, the openness, the heterogeneity of the city. People has gathered in cities as long as civilization has existed. Cities are civilization.
I have lived in London, Barcelona and Stockholm. Great cities on their own, with good public transportation, services, restaurants, that feel safe, alive, and a bridge between past and future. But, your experience may vary depending on the quality of the cities you have lived in.