The debate over the discernible and significant value of 4k v. 5k is reminiscent of other analogous topics.
1.) The higher end automobile experience - a friend once gave me a ride in his Mercedes (I'm a Toyota Corolla man these days) and went on about how it had low road noise. This was even more pronounced later when he gave me a ride in his Tesla. Yes, yes, very nice...but not a big deal to me.
2.) Audiophiles and 'vinyl,' and higher end sound systems. I don't get it (and save a lot of money as a result).
3.) Foodies.
4.) Noise in digital photos. If it's not glaring, some don't readily pick up on it, and for others it may ruin the shot.
For whatever reason, some people have a greater than average ability to discern subtle (to others) differences and their experience is meaningfully impacted. The problem is, until you experience the subject yourself, it's hard to know where you fall in the spectrum of response.
And to be fair, there's an arbitrariness in drawing the line on what's 'good.' Right now, a 5K 60-Hz 27" display is excellent, right? Okay, let some vendors put out 5K 120-Hz 27" displays. How long till people are lauding them, and talking about what an inferior experience the 60-Hz is?
How much more are you willing to pay for lower road noise, very realistic sound, a steak Gordon Ramsey would love, a high ISO sensitivity camera or a 5K monitor?
A lot of people are happy driving Corollas or Civics, listening to non-vinyl music, fast food, iPhone camera work and 4K monitors.
What makes you happy?