The PPI of the 6s PLUS (Full HD on 5,5 inch, 401 PPI) barely acceptable??? Wow, now it's really getting really pretentious ... Or the psychological effect of an even higher number on paper does manipulate your mind way too much. We can argue anytime about display quality differences - LED, OLED, whatever – but when it comes to PPI, your eyes simply aren't able to see the differences anymore at this level. Objectively speaking, it simply isn't possible.
Some people can still discern jagginess, if not necessarily individual points, at 400 PPI.
Be thankful you are not one of them, so you can save money and still be happy with less expensive displays.