Sofia, like any telescope, is a tool that we use to learn about the universe. It has a cost to keep running, and that cost needs to be weighed against alternative uses for that budget.
The primary benefit of SOFIA over ground-based telescopes is that you avoid a level of interference and distortion from our atmosphere. Space-based telescopes offer the same benefit, with the additional benefit of not subjecting the optics to turbulent air. With the advance of adaptive optics on ground-based telescopes and with private space companies competing to bring the cost of a launch down, there are probably more cost-effective solutions to the problems SOFIA was designed decades ago to solve.