> what basis is there for universal healthcare other than 'we should help each other'?
Uninsured, underinsured, or people with high deductibles are much, much more likely to let a health issue linger for much longer than someone who gets free (to them) healthcare. If an issue lingers, getting worse and worse, it may eventually reach the point where it forces them out of the workforce; they stop paying taxes, they stop purchasing commercial goods which drive the economy, they stop buying new cars, etc. (on an individual basis, this is rare, but compounded by 40 million or so people it adds up to a lot of people not performing economic activity who would otherwise be making and spending money)
Free-for-me healthcare is better at providing preventative medicine than out-of-pocket healthcare. Preventative medicine is cheaper in absolute terms over the long term that remedial medicine, even if preventative medicine is much more common that remedial medicine. If remedial medicine removes a person from the work force, that costs the entire GDP a worker.
It is better for all of us -- in purely economic terms -- to have a worker wearing a government issued back brace, taking government issued anti-inflammatory drugs, and participating in government sponsored physical therapy than having a non-worker have back surgery on an insurance plan that spreads the cost among all of that insurer's other customers instead of producing widgets in the factory.
I might not speak for that many people on the left, but I personally support anti-poverty programs as a general rule not because I care about the plight of humanity or whatever, (fuck 'em) but because taxing people and spending those tax dollars on anti-poverty programs improve my overall quality of life more than just letting us keep the money. Every poor person, every homeless person, needs to have tax dollars devoted to them in terms of police and emergency services. Two years ago someone smashed my car's window, stole a $20 backpack with $40 worth of gym clothes, costing me a $500 deductible to replace the window and fix the e-brake. Pretty sure if that asshole had a better education, was given free tradeschool or college they wouldn't have smashed my window.
We, as a society, have decided that we'd rather have a pound of cure than an ounce of prevention. In a dozen ways. And that's fucking stupid.