Not to justify it, but it kind of makes sense. Many of Bernie's supporters are finding a reason to participate in politics for the first time, or feeling cared about by the system for the first time, and when something threatens their new champion they feel like they themselves are threatened.

This definitely speaks to a wider phenomenon in society, where a lot of people had lost hope and are just now finding some, whether through Trump or Sanders. Taking that bit of hope away could be very destabilizing. There's also some interesting human nature in there, regarding conflating one's identity with one's beliefs.

On the back end, rather than Aetna shelling out money to the hospital so that you can get treated for free or cheaply, the government would do so - just like it already does for Medicare and VA.

So it is not $18tn additional burden - it is a similar burden, but not going through the hands of private insurers who take a cut.

Insurance is one of the things that a government can be very good at. It's not an area of great innovation, so the benefit of having it be a free market is relatively limited, and negotiation leverage is very important, so size is very beneficial. The natural tendency of the private insurers has been to consolidate and to look more and more like Medicare in terms of size. Except they also have a profit mandate, whereas Medicare doesn't, so they take a percentage of all spending and suck it out of the system. That portion isn't helpful to the users of the system.