Math isn't opinion. The 1+1=3 example is really no different to what's being said here about PPI. More pixels in any given area create a sharper and more detailed image. This is just scientific and mathmatical fact, there is no opinion about it.
Of course, there is no certainty the human eye will always see it... the difference in PPI obviously factors in and it's extremely unlikely that anyone would notice the difference between, let's say for example, 60 and 70ppi. A 27" 1440p monitor with 109ppi vs a 27" 4K monitor with 163ppi however... if someone can't see the difference there then there is something wrong with their vision.
Image quality doesn't mean all that much to everyone though, and obviously we game in motion, and the fluidity and smoothness of that motion is essential to the experience. Also, some people are far more sensitive to motion speed than others, which is why a 60Hz monitor to one person is more than adequate, while to another it's abominable and virtually unplayable. This is all separate to the point though... higher PPI will always equate to sharper and more detailed imagery. The only 'opinion' that enters in to that is whether it's important to someone or not and how much it effects their gaming experience... that's entirely subjective.
What I actually implied was it’s harder to see the difference on a 27” monitor, less so on a 40”+. I wasn’t even disagreeing about PPI.
Everyone’s a bit touchy in here today!