we were all hunter gatherers 20 thousand years ago and we're designed to focus at a 20 yard hunting range
I'm not sure that's entirely valid. Twenty thousand years ago we had all died by our mid twenties. A lot of eye (and other problems) come from living well beyond our design lifespan.
I have been looking at computer screens for many hours a day for the past 20 years and I just got through my Class 1 with some bifocals I bought in Boots for £12.