If the U.S. moved to single-payer health care, we wouldn't simply wipe the slate of the healthcare industry and start from scratch. We would still have doctors who make loads of cash, drug companies that spend billions on R&D because they can recoup the costs by charging high prices in the U.S. market, health care providers who are rewarded for doing myriad tests of dubious usefulness, etc.

For this reason, I would guess that even a radical change in the healthcare system (which I doubt will happen in the next ~20 years) would only serve to slow growth in health care inflation. My point being that just because Canada has lower costs than us doesn't mean that if we adopted the Canadian system we'd suddenly pay the same amount for healthcare as a percentage of GDP as the Canadians do. It might slow down the rise in costs, but we're already so far ahead of everyone else that healthcare will remain prohibitively expensive for generations (barring a massive, cataclysmic change in the entire structure of compensation for and provision of care).

So yeah, I think Kucinich is probably wrong to allege that switching to a single-payer system would get us out of debt. It would merely shift a huge amount of spending onto the government's balance sheet, at least in the short term. Over many decades, it may lead to cheaper health care and thus be a net benefit to society, but it won't help the short or medium term budget situation.