"To conclude, they have had us thinking that resolution is all that matters, when in truth that only matters with regards to the source material. What really matters with display tech is the PPI."
Resolution and Screen Size matters, that's what the industry tells us, and that's what is correct. PPI is simply an amalgamation of both these two figures.
To always go for a higher PPI display ignores the factor of perceived image size, which is based on size of display and relative distance, of which their is an ideal size. The DS Lite had a 102PPI display, a 4k 50" TV has a 88PPI display, does that make the DS Lite's display higher quality? Nope.
Basically you choose the resolution you want, and the size of display that fits the distance you will be viewing it from, and as these two factors are already the sole source of where the PPI comes from, you don't need to worry about it. Resolution and then matching Size to Viewing Distance is what matters when choosing a display.
PPI is not necessarily the deciding factor even at the exact same distance. A 4" 1080p phone will have a higher PPI and thus image-quality then a 1080p 5.5" phone that will likely be used at the same distance. However perhaps you want a bigger perceived image size over the drop in perceived quality.
