In this case, IMHO, the pilots had become complacent and lax in their navigational rigor because of the fact that GPS makes a lot of the slightly less automated systems unnecessary, and people get used to that workflow.
If the flight crew had been looking at their map and using it to inform them about the mountains in their path, fixing position by the VOR and DME, or even just a stopwatch and compass heading, they would have easily known they needed to be at a higher altitude. Not being precisely sure of their exact position, they should have at least been maintaining the minimum safe altitude for the area they knew they were in.
But unfortunately, people get so used to looking at the shiny screen giving them effortless situational awareness that they forget all of the rules and habits that kept pilots alive for the last century before glass cockpits were a thing, and they forget that mountains are the tigers of the air, always waiting to catch pilots unaware.
This is a useful cautionary tale not only for pilots, but increasingly for humans in general as we become accustomed to a new kind of instrument that allows us to offload a great deal of our cognitive focus to a tool that may sometimes fail.