Pace:
Why do Jet owners go to vast expense fitting TAWS etc to protect pilots from mistakes they may make?
Because an American Airlines crew managed to really muck it up on descent into Cali, Colombia in 1995 resulting in the esteemed inventor of GPWS proceeding to the next step, a terrain database EGPWS. In turn, the FAA (and I presume other state regulators) mandated it be installed in air carrier airplanes over the objections and whining of some airline managers.
At the time a large percentage of the fleet that did have LNAV did not have GPS. So, the vendor offered a GPS receiver for the EGPWS for $1,500 to make the EGPWS positioning independent of all-too-often unreliable DME/DME determination of ship's position. But, in the U.S. at least, the FAA did not mandate the GPS receiver option, so one major airline refused to buy the GPS option at a measly $1,500 per airplane.
I'll let you reach your own conclusion about that. I long ago reached mine.