Randy Barnett notes that the Obama Administration has conceded that if the individual mandate is held unconstitutional, "provisions preventing health insurance companies from discriminating against people with pre-existing conditions would also be invalidated by the court because the two are inseparably linked".
To me that doesn't seem like a big concession. First, it's obvious that those regulations would not be economically viable in the absence of an individual mandate. Second, were the Obama Administration to argue otherwise, the health insurance industry would launch a full-bore assault on the bill both in the courts and in Congress, and they would (for once) have a valid argument to make.
Frankly, given how many concessions the Obama Administration had to negotiate to get a bill that the various health industry players wouldn't torpedo, it seems less reasonable to assume that the Dems "forgot" to include a severability clause as opposed to having deliberately omitted one in order to prevent an outcome where a court upheld the regulations while striking the individual mandate. The "need" for a severability clause is largely overstated by opponents of the legislation and, to put it mildly, the inclusion of a clause that would have stated "If the individual mandate goes, so do provisions like 'no discrimination based on pre-existing conditions'" would not have been politically popular.