I think Zubrin has made a good point:
building a heavy-lift launch vehicle makes little sense unless you have a mission for it. (Of course we all know what he thinks the mission should be - Mars in 10 years.)
Even if NASA produces some kind of an HLV that will satisfy all the political interests attached to the programme, it will be hugely expensive and critically underutilized. In order to lower the cost, you need to be producing a reasonable number of these rockets per year and keep sending them up. Anything else is a waste of money. If you develop an HLV just to have an HLV, and then launch once every couple of years while still paying for the armies of workers, engineers and bureaucrats who maintain the production line, launch pads and whatnot until you think up a suitable mission, the whole thing will end in a disaster (and most likely, cancellation).
NASA needs to have some concrete goal, it must know what it is trying to achieve, and then procure all the necessary hardware needed for successful accomplishing of that mission.
(Oh, and the crap about whether you can build an HLV in 6 years - it took 4 to build Saturn 5, and back then those guys were literally starting from scratch. Today, NASA has all the building blocks it needs. If it received a dose of common sense combined with an antidote to politics, it could have an HLV on the launch pad in 2 years with the current budget.)