NASA HQ set a series of engineering milestones before the SIM Team in 2000. Those milestones were met, or exceeded in each case.
Each of these milestones was designed to "prove" that SIM would be able to actually chart stars with an unprecedented accuracy. That precision is the key to achieving SIM's multiple goals, including the detection of Terrestrial ExoPlanets.
The 1st Four Milestones were to be met before SIM would be allowed to proceed to Phase B in 2003. The SIM team successfully achieved that. By 2005, the SIM project had checked off all of the 8 milestones set before it. Since 2005, the team worked on reducing engineering risk further.
What we are faced with, in these times of extreme budget pressures is a choice of whether NASA should, once more, walk away from a partially complete program.