Before SW failure was mechanical failure. And that's not gone away either.
But you can usually detect mechanical problems early: for example, wear or cracks in metal parts. Software may work perfectly for ten years, then finally hit the rare bug that causes it to fail for no apparent reason. Worse, every instance of that software may fail at the same time all over the world (e.g. the various leap second bugs). Gears don't even know about leap seconds.
The other issue is that a third party can examine all the mechanical parts for cracks, and tell you there's a problem. A third party usually can't examine the software that runs those parts, because it's closed source. They can only test it as a black box.
There's a truly scary document online from one of the software guys who was given access to Toyota's software as an expert witness for the 'unintended acceleration' trials. Some of the things in there are quite mind-boggling, but no-one knew about them because they had no access to the software.
Software has definitely made many things far more reliable. But it's also replaced many predictable failures with unpredictable ones.