"Although NASA is proceeding with development of the SLS, a number of outside panels have questioned whether NASA can afford to build, fly and, sustain the expensive program, especially with projections of low flight rates of one launch or fewer per year. The biggest concern is that the rocket is so expensive to fly it precludes a meaningful exploration program within NASA's existing budget."
Keith's note: With the cuts to both SLS and Orion in the Administration's FY 2017 budget you can expect the same food fight with Congress to pick up where it left off last time. And as was the case before, Congress will go after Commercial Crew and Cargo, Technology, and Earth Science to put SLS and Orion back at the level Congress wants. Of course, election time will soon skew everything and the chances that there will be a formal budget will drop. The net result is that NASA will not know for certain what its budget will be and this uncertainty will cause launch dates to slip to the right. With these slips the overall cost of the SLS and Orion programs will increase - and commercial crew will take longer to happen than might otherwise be the case.
Naturally, the next Administration will stall for time and eventually appoint a blue ribbon panel to write a report and the cycle will start all over again. Their conclusion will be that NASA has no plan (and that it needs to hurry up and develop one) and, by the way, NASA cannot do all of the things it has been tasked to do under a budget that does not grow. Considering that all of these arguments are set to occur under a NASA budget that is likely going to stay flat, nothing will change since no one will give up pushing for the things that they want NASA to do. The inevitable result will be that NASA will end up with a launch system that will have nothing to launch on the imaginary #JourneyToNowhere.