NASA designed the Space Launch System (SLS) rocket and Orion crew capsule to provide the capability to send humans into deep space. Congress mandated the creation of SLS in the 2010 NASA Authorization Act, though they didn't specify how exactly SLS and Orion were to be used or precisely where and when NASA should send astronauts. It makes sense to take advantage of these new capabilities that are years into their development. What kind of human spaceflight program can you put together using the current infrastructure NASA is developing? Orbit-first is one potential answer.
Canceling both programs now and developing the political consensus around their successors would require a massive restructuring of several major field centers and likely face considerable congressional opposition (see the 2010 battle between the Obama Administration and Congress when the White House canceled the Moon-focused Constellation program). It seems to us that directing both the SLS and Orion to support the human exploration of Mars is a better use of time and effort than fighting again to cancel them.
This isn't to brush aside the major concerns related to the cost of developing, maintaining, and flying both SLS and Orion. These programs must find ways to increase their flight rate and decrease their overhead costs to make these programs sustainable over decades. Using SLS to launch robotic probes into deep space (which dramatically reduces their transit times) is one way to increase the launch rate of the SLS during the 2020s, but the other issues remain.
Many commercial enterprises are now attempting to travel in space using different technical and financial strategies than companies have traditionally pursued. As these companies are able to prove their new capabilities, NASA will find itself presented with new opportunities to enhance or refine its human spaceflight strategy. It will be important to understand the differences in the capabilities of new spacecraft and launch vehicles as they become available, and also to accurately assess when these capabilities will reach a level of maturity at which the risks of the vehicles are fully understood. Nevertheless, as we discuss in the report, NASA should actively pursue the best assets available to successfully enact all the phases of a human spaceflight program to Mars, but to do so the space agency needs to better define its goals and strategy.