The thought is a large percent of population is going to get regardless. Like you said.
Say 50 million people get it and 10 million need some kind of treatment (totally made up numbers to use as an example)
So it's better to treat those 10 million people in smaller groups broken up over time rather than all 10 million rushing in at once.
How long to lock down to spread it out?
Longer than they're saying.
Some will stay out and get it now. And that's good. Because otherwise, it just means one big rush later.
How long would it take the medical system to deal with 'only' 50 million (and only 10 million severe cases)? Years? And what if it's 150 million, or what if people can get re-infected by mutated strains like the flu?