Shouldn't you expect Zoomers to be more than double millenials? They're 20 years younger, which means money is 4x cheaper.
For a current 20 to 30 year old, they should expect SS benefits to come at a higher age, probably 72 or even 75, and whatever benefit amount they get will buy less than what it buys today. And they will have to shell out for better quality healthcare (i.e. a couple decades ago, they might have seen a doctor, but today, they see an NP/PA, and in a few more decades, they might not even get that).
The big expense is insuring against loss of income between age 50 and whatever age government benefits start. Most people will never make their way up the income ladder again, so they need to make sure they have adequate savings by then, otherwise they are cooked, and those are the ages that healthcare expenses start adding up.