For starts, you've confused Medicare (health insurance for people over 65) and Medicaid (medical welfare). Gummint likes doing that, for some reason (Social Security and Supplemental Security Income is another example of two income-transfer programs with confusingly similar names).
Whatever the long-term net cost of government-provided universal health coverage, you can't seriously argue that paying for it won't be staggeringly expensive. The costs of both Medicare and Medicaid have ballooned far beyond what anyone predicted when the programs were first implemented - not just in dollar terms, but as percentage of the total federal budget.
Anyway, we're getting away from my main point: there are a lot of proposals for spending in the Democrats' plan, but very little about how to pay for them.
I most certainly can argue that the costs won't rise. Watch me do it.
Let's look at the situation of our fellow poster, Fowlsound. Fowlsound has a potentially lethal form of cancer in his spine. Currently, he recieves government aid to help provide him treatments which reduce the chances for a relapse. Now, in monetary terms, a relapse would mean tens, or even hundreds of thousands of dollars of medical procedures, if you want to ignore the human dimension of suffering with cancer. The way our system is currently set up, if Fowlsound's income rises about a modest level, his government assistance for medical expenses dries up. If he suffered a relapse, he would then get his treatment (as his ability to pay would rapidly be exhausted), paid for by the government in the amounts of hundreds of thousands of dollars.
Let me summarize.
We either pay thousands of dollars for prevention and treatment, or we ignore the problem, and then pay for hundreds of thousands of dollars worth of care.
Again, a penny of prevention is worth a pound of cure. Now, a strict conservative might say this situation we presently face is a reason to abolish all forms of government medical assistance entirely, but frankly, I'm not that cold blooded. It's difficult to dispute that workers who are healthy, and have an expectaion of being cared for if they become ill, are more productive. For one thing, if a child in a family comes down with a preventable condition that, beccause of our tightfisted poolicy, went untreated, that child's entirely family may be economically harmed by our unwillingness to help prevent that disease.
Even from a purely economic perspective, I don't see how one can say that government assistance for medical benefits is a bad idea, I would tend to think that the nature of the debate should revolves around what kind of program to implement, and how much to allocate, not whether to do it nor not. I hate to beat a dead horse, but since a great many medical conditions can be treated relatively inexpensively when caught earlier, it makes sense to focus funding on regular medical visits, routine testing, and screening for potentially deadly (i.e. costly) conditions).