1080p phones are not absurd...

I keep earing that FullHD phones are unnecessary and are a waste of battery life and money because their DPI is above 400ppi when the normal human eye can only see up to 300ppi and so on...

Sorry to disappoint but thats not (entirely ) true , at a certain distance (about 11.34 inches/ 28.8 cm) that its true , but even a 1ppi screen is "retina" at a certain distance (286 feet / 87 meters) .

but a phone it's used many times at a smaller distance than that (at least i do use mine at smaller distances)..

Hijacking the equation from another topic , [VA=dot pitch/(2*tangent(rad(((1/60)(20/20))/2)) ] , you get that a screen with 400dpi is "retina" beyond a distance of 8.60 inches / 21.84 centimeters ,

The DPI it's not what defines if a screen has the max resolution that the human eye can detect, every single screen is "retina" at a certain distance ; but if you throw in the viewing distance for with the device is intended then you get the ppi in that case perceptible to the common human eye.