Medicare has a total enrollment of approximately 69 million people, while Medicaid has around 83 million people. That is 152 million people. We already have socialized medicine we just run it poorly and don't apply it to people that can pay.
Moving our system to 340 million people + letting our corporations out of paying would put the US into an economic death spiral. US corporations would love this plan. But at 340 million... I don't see doctor visits but once every 2 years -- many would just die waiting for appointments.
Medicaid is apparently 77MM including CHIP. The underlying compromise in the system that you're describing is sane: people's health care costs rise dramatically and unpredictably at retirement age, just as their ability to pay plummets, so socializing health care at that point makes a lot of sense.
Moving our system to 340 million people + letting our corporations out of paying would put the US into an economic death spiral. US corporations would love this plan. But at 340 million... I don't see doctor visits but once every 2 years -- many would just die waiting for appointments.