Id82 - Without seeing the arguments against Sander's plan, I can still make a guess that they are not factoring in that they would no longer be paying for health insurance directly or from payroll deduction. Their health care would be a big chunk of their tax rate increase, at lower cost than their private health insurance premiums. With low or zero deductions, mind you.
For example, I had a slow business year and had to drop my health insurance under the ACA, because after trying to keep it by raising my deductibles to barely realistic amounts, I still could not eat that premium every month. For a more average middle class single childless person my age, paying my insurance without ACA (and still with shitty high deductibles) would cost about 550 a month, for a younger person in their thirties, about 360 a month. That is for insurance from a reputable provider, but with painfully high deductibles. Hundreds a month to be effectively underinsured. It's a fucking racket.
Any version of anybody's monthly healthcare costs, whether it's "hidden" because they are never told how much of their actual pre-insurance salary went to their "company provided" insurance", or if it's shown on their paystub, is more out of their pocket than Sander's tax increases, due to the world-wide demonstrated economies of scale inherent in nationalized health insurance programs. So, tell them to multiply their actual or estimated health insurance premium by 12 months, remove it as a cost, and they will see that Sander's tax rate increases do not hurt their bottom line much - if any, and possibly they save money - all the way up to mid 6-figure annual incomes.
Feel free to copy/paste my post on that forum, just leave my user name here off of it.