The virus is a death sentence for a small percentage of those infected, yes. Or at least they live with permanent damage to their lungs and must carry an oxygen bottle around for the rest of their lives. Because they have pre-existing conditions, because they're old, because their immune system is compromised, because they're smokers (!), because they're getting a cancer treatment, because they have diabetes (!), whatever.
Let's say, for simplicity's sake, that one percent of all people die. It's really irrelevant for the purpose of the discussion if we're using one percent, five percent, or 0.1%.
The R0 of this virus is somewhere close to 3, meaning while you have the disease, you on average infect three other people.
You infect three, first step of the spread. Each of these three infect another three. That's nine new infections in the second step. Nine people now infect 27 people in the third step. Those 27 infect 81. And so forth.
By the tenth step, you have ~60,000 new infections. By the 12th, ~530,000. By the 17th, ~129 Million.
It happens slowly at first, then suddenly all at once.
And now a percentage of these people dies.
If you have millions of people who are infected over a short period of time, then your mortality rate goes way up, because you don't have enough hospital beds, respirators (which often cause severe and permanent lung damage in COVID-19 patients!), ambulances, doctors, nurses, etc. Now you're getting a mortality rate of, say, 2%, or 4%, or 10%. That's what happened in Italy, and other places where the disease spread quickly without initial containment.
If you only have hundreds of thousands of people who are infected over the same period of time (because the disease is spreading more slowly), then the health care system doesn't get overwhelmed. It can treat all severe cases, save most of them, and you have a mortality rate of, say 0.1%.
I really don't understand why people are even debating this. There are enough examples, current and past, where exactly this happened. Be it the Spanish Flu, or the initial weeks of the outbreak in Wuhan, or the now slowly improving situation in Italy/Spain/France, or the current situation in New York City.