Facebook and Google sometimes use the expression "one in a million" to describe edge/exception cases which occur every day when you serve many users. The density of NYC is a real-world equivalent, there are continuous exceptions that cannot be centrally predicted and must be micro-locally responded.
There has to be non-zero social interaction of young people who are least at risk, and can recover most quickly. Otherwise the only people being exposed will be essential workers. After recovery, these young people can volunteer to train/run errands for hospitals, reducing load on the system and keeping them active.
In times of crisis, civilians working with each other can make a difference. Wish they had used the language "physical distancing" since people can still coordinate socially (including online).
Instead of the California approach, where you say "don't this, except for reasons 1 ... n", he's attacking the demand side. You can go out, but you don't have anywhere to go.
All of the problems get worse if you make people panic. My buddy who lives in northern upstate NY was just telling me about all of the people "bugging out" and taking their RVs up to the Adirondacks. If you tell people it's martial law, people will flee NYC Metro for wherever.
The other thing is that people will always be out. How do you deal with homeless people? Arrest them? How do you deal with people who claim to be medical people? How do you identify Trader Joe's employees? Are the police going to setup roadblocks? How do the police maintain a rosette stone of who is worthy of travel and who isnt?