I'd disagree. A lot of potential Model S buyers (not necessarily active on this forum) including myself are going to be very, very price-sensitive; most of us are seeking to buy a car that's twice - or more - as costly as any car that we've ever bought. The federal tax credit is a tremendous help!
The 40kWh pack is what it is (not too much over a Leaf's range, no supercharging, etc.) and may not be attractive enough to a lot of cost-conscious buyers who might settle for a Leaf, Focus Electric or Volt instead. That leaves the costlier tiers which are made that much more affordable (relatively speaking) with the aid of the tax credit.
Every new radical shift from the status quo needs some sort of nudge from the Govt. (the only entity that can afford to do so without necessarily worrying about the bottomline) and it's up to the politicians to defend it.