Well of course if everyone gets infected deaths would be high, but what makes you think everyone is going to get infected? How do you know this is going to infect 80-90% of people? And the economy will collapse if everyone is told to stay in doors for however long never-mind the hospitals. How long are we meant to self-isolate for? I don't think staying indoors for a few months is going to cut it and even that would be disastrous. If it's as extreme as you say then why isn't there a full on lock-down for everyone already? Telling people to just not go to the cinema or sporting events isn't going to do anything if everyone is still going to work etc. How often are we going to have to self-isolate for before the coast is clear? Is this going to happen everytime a new form of SARS appears?
People keep explaining to you why this needs to be done but you aren't listening. It isn't long term going to stop infections from spreading to nearly everyone, but it WILL SLOW IT. This gives the medical system and the supply chains critically needed time do adjust and absorb the overwhelming demand, and hopefully prevent a total breakdown of the global healthcare system.
The issue is not that this disease is just so fatally dangerous, but that is is just dangerous enough, and mostly debilitating with many requiring long hospital stays to treat. This disease has a unique combination of long viability on surfaces, being airborne, long incubation periods, passive carriers, and high rates of infection that is basically like a checklist of all the things you would put into a disease you intend to cause a pandemic.
Again, the rate of fatality alone is not the major threat, the number of people debilitated and requiring hospitalization all at once all over the globe is the real threat. Quarantines slow that infection curve down, and hopefully gives the system time to adjust and compensate for this huge wave of hospitalizations that is about to happen all over the world.