I haven't run into many screen issues, insurmountable ones that is. You're going to have 4 problems:
1. Bad capacitors, in which you replace them and the screen starts working again
2. Vinegar Syndrome, where the LCD polarizer layer's adhesive fails and causes problems. This can be replaced, although I've never run into it.
3. Tunnel Vision, an issue only on active matrix monochrome and grayscale displays, where the corners of the screen will darken, creating a vignette effect. This can be somewhat fixed by baking the LCD in the oven (yep).
4. Bad pixels/dead lines. Can be caused by bad connections, failed driver ICs, age, a broken display cable, etc. It isn't nearly as common as you'd think though.
So anyways, most screens still work/can be fixed with a bit of work.
As for "old iOS", it's pretty much an age thing. I'm a lot younger than a lot of people in this hobby, and so for me the iOS 6 days do feel like a long time ago, and I do feel nostalgia over them. I can understand why this wouldn't be for everyone though.