The reason why this plan doesn’t work is because 20% of people who get this disease (regardless of age bracket I believe) require hospitalization and a good number of them require an ICU bed. There simply aren’t enough beds or health care workers to address the demand shock that would happen without social distancing, which means people dying at home waiting for a bed.

What is the alternative? Shutting down doesn't work either, because it's not possible to shut down until cases worldwide go to 0, so won't the outbreak just start again after things reopen?

The whole reason for social distancing and #flattenthecurve is to delay mass infections long enough for the scientific/medical establishment to develop a vaccine or, failing that, effective treatments. There's an understanding among policymakers (though perhaps not in the general public) that either everyone's going to have to get it, or a vaccine's gotta be developed. The problem is that with a 2% mortality rate, "everybody's gonna get it" is 140 million deaths, the largest mass casualty event in history. This is a lot like one of those movies where an asteroid is hurtling to the earth and a bunch of random oil roughnecks need to detonate a nuke inside it - it's a hail mary, but not doing anything is not an option. The hope is that the biomedical industry can develop a vaccine, effective treatments, or at least a lot more ventilators and hospitals to blunt the impact before everyone gets it.

It's not just old people who are at risk, BTW. With medical treatment the death rate of under-40s is about 0.2%, 30x less than the ~6% of over-65s. However, young people are still hospitalized at roughly half the rate of old people; if the hospital system is overwhelmed we can expect a death rate of ~5% of young people. That's about a 10 million young people in America alone. That's why it's so important to delay the epidemic long enough to build enough hospitals and ventilators to treat everyone.

> The whole reason for social distancing and #flattenthecurve is to delay mass infections long enough for the scientific/medical establishment to develop a vaccine or, failing that, effective treatments.

This is extremely unlikely unfortunately, most flattened curves end up overtaking the populace at large - the more crucial element to control is the proportion of the population that's sick at any given time to avoid overwhelming healthcare or logistic resources.

The Brits had that plan for a while but gave up after a few days. I think they realized that the numbers don’t work.

I think the factor you're missing here is the hospital bed ratio. When folks, even healthy folks, get Covid-19, it can go badly - so all this social distancing is to let it spread really slowly through the populace - maybe so slow we get a vaccine, but probably not... And while it is spreading we'll ideally have enough healthy people to actually care for the sick ones because if you and everyone around you is bed-bound then people can die from pretty trivial complications of the disease including just starving to death.