GPS can fail for a variety of reasons, not all are "technical issues
which will eventually be solved". Interference is a growing issue for
example. Batteries die, wired connectors stop connecting, and
sometimes electronic components just plain stop working. Don't get me
wrong, I like the GPS, I think it's pretty useful and cool as well.
But, being a belt-and-suspenders man, I like having and knowing how to
use backups too.

While the PTS may not require doing calculations by hand, examiners can
and do "fail" GPS devices and expect the applicant to perform well no
matter what. As would your passengers if this happened in real life.
-Malcolm Teas