Resolution for one thing. The XR is barely over 720p. Also you go from an OLED screen down to LCD.
The Plus phones have had 1080p resolution always (2014). The X is sharper/higher PPI than the the Plus phones too.
Better cameras as well in the 10
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XR also has a better camera, better speakers, stereo sound recording in videos, better CPU, GPU, bigger screen, LCD won't have burn-in in the long run, better battery life, better selfie camera, etc.
It's an upgrade, not a downgrade.
If it was an upgrade it wouldn’t cost less, Apple didn’t make it to a trillion selling cheaper models that are better than the flagships..