I feel like the demonstrated effectiveness of quarantining old people is the reason you can't do that. It sounds great on paper, but it turns out that homes are staffed with lots of casual workers who work multiple jobs, and keeping covid out of them is near impossible.

Could this be addressed? Sure. Should it in future? Probably. But we don't get a whole lot of tries when there's a pandemic going around. It didn't work at first, how many attempts do you have to give it before you say "we need to solve that next time, right now we need a different solution"?