I think it's unknowable. There are too many variables including currently out of control cost increases and the possibility that the government will completely change the landscape.
But for what it's worth, I started my HSA in the start of 2008 and (including the employer match) it's over $7,000 today. Once I have at least two years of out of pocket maximums (currently a total $8,000 for $4K a year) in cash, I'll start looking at investing the rest of it in something like a balanced fund.
Depending on how the universal health care initiatives play out, I'd still think you could use the HSA for any copays or deductibles that remain, or for OTC medical supplies and OTC medicines, and dental/vision care. And I'd assume you could still withdraw it for any reason without penalty after age 65. I don't see anything more sinister than that (i.e. I don't think any "universal" plan would confiscate the account balance).
"Hey, for every ten dollars, that's another hour that I have to be in the work place. That's an hour of my life. And my life is a very finite thing. I have only 'x' number of hours left before I'm dead. So how do I want to use these hours of my life? Do I want to use them just spending it on more crap and more stuff, or do I want to start getting a handle on it and using my life more intelligently?" -- Joe Dominguez (1938 - 1997)