> Or to look at this from another angle: 'extremely' bad is inherently relative. And taken relative to other common ailments people that old content with, I don't think 15% chance of death qualifies as 'extreme'. If 15% is 'extreme' then what adjective would you use for a cancer with a 50% mortality rate? "Super-dooper-uber extreme"?
You're missing the time frame. Getting cancer is a threat that many old people face but that's spread over decades.
With COVID-19, we face 70%-80% of the population being infected within a much shorter time, possibly a year or less (depending on how well or bad the delay measures are).
Yes, people die at that age for many reasons. But here, we have a potential death threat that is avertable and, most importantly, that affects the whole of society, not only old people.
Don't think that only old people get it and need medical attention. Lots of young people also need medical attention, even if they don't die from it. Ignoring long term effects from having caught it (as we don't know them yet), this alone would disrupt society. And then the secondary effects of people with other medical conditions not being treated as they are triaged.
Modern society doesn't need a lot to be disrupted. Look at how disruptive 9/11 was.