Actually, that is what Bernie is now doing.
Medicare was intended to be for all. Initially launched it covered the elderly
Bernie wants to change that so that everyone can enjoy Medicare
He also wants to make it better by adding dental, eye, and home care coverage
Based on what he has presented it appears there will be no premiums or need for supplemental insurance to pay medical expenses
So, your wish has been granted, Bernie is proposing to improve the existing Medicare program such that it becomes an effective Medicare program for all
Both Sanders AND Warren have been repeatedly asked
"what about if people don't want to lose their choices?"
This is hilarious because since when you do get a choice right now?
A small sliver of VERY well off people get a wide range of choices when covered by current private sector insurance, but most people get told they can choose between one, two or maybe three doctors, and a couple of hospitals.
Everyone else is "out of network".
Not really much of a choice.
They also get told what drugs are covered and which aren't, and in the end, even if they're paying for 24 kt Cadillac policies, they sometimes STILL get denied important or even life-saving procedures anyway.
It's happening right now as you read this and it's been happening.
So the real question is, do you like your doctor and do you like your hospital, or do you like your insurance company?
I seriously doubt you will find more than a handful of people who will say that they love their insurance company.
"Oh I love CIGNA...I love Aetna, I love United Health."
No, people like their doctor, they like the clinic and the hospital they go to.
A single payer system isn't going to take away your doctor or your hospital, because the whole point is, there won't BE anymore
"in network/out of network" nonsense anymore...you go to whatever doctor or hospital you want.
The whole reason Obama wound up making a mistake in judgment over
"If you like your doctor you can keep them" is BECAUSE the insurance industry screwed that whole aspect of the ACA up.
That's what happens when a "moderate" Democrat with good intentions tries to work with Republicans.
And for the very wealthy who want to fork over for private practice, I GUARANTEE you that will still be available, just like private schools still existed even after the debut of public school systems.