Earlier quoted context omitted.
It is worth mentioning that if the Senate bill version of Medicare for All becomes the law of the land, then previously employer-funded healthcare becomes one less thing for union leaders to have to negotiate with employers.
Why? It just raises the bar for the unions: negociating co-pay, dental plan... Plenty of healthcare things will not be covered by Medicare. Unions will negociate about this.
"All Americans would have coverage for comprehensive health care services, including hospital stays; emergency room visits; doctor visits; substance use disorder treatment; dental, vision, and mental health services; long-term care; and reproductive health care. Depending upon income, prescription drug cost sharing would be capped at $200 annually." -- https://www.webmd.com/health-insurance/news/20191120/medicar...
Obviously, there might be some edge cases. This is where private insurers could still fill a role. I can't imagine that extra/supplemental private insurance could be too expensive, if it had to compete against a unified 350 million+ person insurance pool.
Edit: It is worth noting that the US is pretty unique when it comes to the separated dental insurance coverage plans and main health insurance plans.