I think you raise an interesting point, but one that ultimately cannot be sustained.
If we attempt to assess the constitutional validity of the scope of government through such a very strict filter, then vast swaths of what is done now would have to be considered unconstitutional. That is one of the problems I had with the argument against Obamacare: if that were to be found unconstitutional, then Social Security, Medicare, Medicaid, housing assistance, most every safety net program, energy programs -- you name it -- would be equally unconstitutional.
But there is obviously no desire among the electorate, except among very small factions, to go that way. The only conclusion that can be drawn is that we have decided, as a nation, to adopt a fluid view of the constitution. And once that fluidity is accepted, then Obamacare fits easily under the same tent.
I might liken this to certain of the treaties signed with native peoples in the 19th century. Taken literally, those peoples may have a claim to territory and rights vastly beyond what they currently enjoy. But there is a sense that the developments of the past 150 years, even if contrary to those literal words, cannot be undone. So too with the fluid progression of the Constitution. We DO have all those government programs, and that cannot be undone. And that is precisely why finding Obamacare to be constitutional makes perfect sense.