When I used my VR headset on my old 5.5", FHD, android phone, I wished the screens resolution was higher.

Again, when using a VR/Google daydream-headset, a high resolution screen is necessary, if you want to minimize/get rid of the visible pixels. For focusing there are lenses in front of the screen, which enlarge the pixels by a lot. To combat this, one just have to pump up the resolution.
A higher resolution logically means a higher load on the GPU, but there is a way to combat this. You don't have to run everything at the screens native resolution. One can browse the web just fine at 720p on their native 4K screen. Want to watch videos in its native resolution? Just change it yourself or let the OS change it automatically, based on what app you are running.
I think cyanogenmod is/was capable of that.
Samsung devices have that option too.

I agree. For everyday usage, it's more than enough. For special uses, a higher resolution screen is "necessary"/more than welcome.

Higher res. screens don't even need to be more expensive than lower resolution screens. The product that is available the most is the cheapest and the one that isn't, is more expensive.
Supply and demand.
Chens slider device is a good example of that. Chen has to pick a easily obtainable 5.5" fhd screen, because smaller screens are more expensive.


See, there is nothing wrong with high resolution screens.
The only thing wrong (currently) with them, is their implementation.
If Chen was to use a 4K screen in his 2nd slider device, you should be able to run it at 360p and save battery, but probably have to scale the OS/apps accordingly, to not get massive icons, buttons etc..