I don't think anyone is opposed to letting people buy private insurance on top of the obligatory single payer. Does any country actually ban additional private insurance?
“ In fact, the bill would outright ban private insurance that provides similar coverage to the new Medicare for All plans after a short transition period. That means everyone with comprehensive employee benefits or a private plan through the Affordable Care Act today would be moved onto Medicare.
[. . .] Sanders has said he envisions these remaining plans covering a handful of items like cosmetic surgery that are left out of Medicare. For everything else, the only option is Medicare.”
It's not entirely clear, but I think the plans that are 'banned' in that case are not "on top of" Medicare, but instead of. I.e. there's a contribution you'd pay to private insurance that you don't pay to Medicare under those plans. Because otherwise... why bother banning them?
I believe the reason they are banned is to avoid a situation where doctors refuse to accept patients who only have Medicare-for-all insurance, and only care for patients who have private insurance.
Since private insurance is willing to pay far higher rates than Medicare, this would lead to Medicare-for-all failing. Doctors already exist who refuse to accept Medicaid-insured patients because the reimbursement rates are low compared to privately-insured patients.
Everything I can read about this plan says it would ban plans “on top of” Medicare, and contrasts it with other countries like the UK, Canada, and Denmark, which do have supplemental private coverage for medically necessary services available.
“ In fact, the bill would outright ban private insurance that provides similar coverage to the new Medicare for All plans after a short transition period. That means everyone with comprehensive employee benefits or a private plan through the Affordable Care Act today would be moved onto Medicare.
[. . .] Sanders has said he envisions these remaining plans covering a handful of items like cosmetic surgery that are left out of Medicare. For everything else, the only option is Medicare.”