When you think about it from a first principals perspective, having multiple touchscreens is better than only having physical switches. When a switch is damaged/fails, you are out of luck. When a touchscreen is damaged/fails, you use the one next to it. On a rocket you do not have the mass or room to have more than 1 of all but the most critical of switches.

There have been quite a few missions that nearly caused death or mission failure directly due to a switch getting broken (Apollo 11, lander return engine-arm switch) or going faulty (Apollo 14 abort switch).

What really matters is that they have no single point of failure (touch screens can do everything switches can, an individual touch screen is not important, and switches can cover abort/return scenarios to protect the crew). For the software, it only matters that its been fully tested, including random bit flips and hardware failure.

From a cost savings perspective, its vastly cheaper to verify that 3 touchscreens are working correctly than the 600 switches they replace.