I don't think that replacing the Social Security tax with a petroleum tax would have any worse problems than the Social Security tax. For one thing, once there is little oil, there will probably be much less income to tax, so the Social Security system will have problems, no matter what we do.

I personally am pretty much a doomer when it comes to the US monetary system - I am afraid it will fail fairly early on, once people figure out about peak oil, and lending institutions stop making twenty or thirty year loans, including mortgage loans. Even the present downturn in the housing industry, if it gets worse, could hit the monetary system pretty hard.

Another thing to keep in mind is that at any point in time, there will be only so much of quite a few things available - oil, food, fresh water.  The current monetary system or some new monetary system can help divide these goods up, but it can't make any more than there is in total.

With all these issues, I think that people should not count on social security, medicare, medicaid, and other social programs. We may luck out and get a little from them, but if there is not enough to go around, social welfare progams are likely to be cut. Private pensions are not likely to fare a whole lot better - they depend on the stock and bond markets to fund them, and will have problems if there are many bankruptcies, or problems with the monetary system.