If you can't see the tiny details, no need for a sharp resolution and even less need for a 5K screen !!!
Actually, retina screens help with presbyopia. I originally thought that it wouldn't matter and since I'm in that age range where I need to deal with bifocals/progressive lenses/reading glasses, I was happy to stick with my old 17" MBP because I thought that physical size was the only thing that would let me fit more content on the screen. But then I got a 15" rMBP for work and discovered that I can easily read as much or more content on the smaller screen as I could on the 17" due to the sharper text. Whenever I go back to my personal 17" non-retina laptop I find everything harder to read even though it is larger.
Sharpness does matter when your reading eyesight is failing...