"Depending on the combination of these premium-driving factors other than benefit level, premiums for a given employer can be expected to exceed the threshold for incurring the tax even for a plan without especially rich benefits. As a result, although the excise tax is often referred to as a tax on overgenerous health benefits, it is likely to be a tax based on factors other than benefit level and beyond the control of health plan members.... There are also many areas of the country where the combined effect of premium-driving factors will make it unlikely that the threshold could be exceeded, no matter how rich the benefit plan."