Your simply wrong about the importance of ppi. iPad Pro only has 264ppi—much lower than the XR—and you don’t see complaints about pixel density on that device. You’re fixated on a spec that very few care about.The LCD screens in Apple devices are known for good color reproduction, especially the newer ones with support for the P3 color gamut. But for day-to-day use, increasing the PPI is usually what makes the biggest noticeable difference in display quality, though obviously there is some variation depending on the person.
In my opinion, no smartphone with 326 PPI in 2018 can be called premium, period. Price it at $649, and it gets much harder to argue about the PPI, lack of 3D Touch, or lack of dual-cameras.
If you want a higher ppi, buy the 8 or the XS. You aren’t going to get a $649 XR until next September. It’s $749, and it’s going to sell massively at that price.