PEI, you have hit the nail on the head. If you do not have accurate in-cockpit slope reference ie RNP-AR, IAN in the 737? or a GS, then the only hope of staying on-slope is by reference to the PAPI (or more basically, "that runway looks very flat"). The fundamental mistake here was that the crew pressed on when they obviously could not see the runway and the PAPI. My takeaway here is it is critical that, before proceeding below the MDA/DDA, the runway and very shortly after the PAPI is clearly visible. As Slast has pointed out, the Canadian rules are "sus" in this area, apparently allowing descent without seeing the actual runway/PAPI.