PCs could choose to retire early with a few thousand gold, but they'd be missing out on a lot, mainly power.
At level 5 you're pretty strong, but an orc war band, an evil lich, or a dragon in your area can still ruin your life on a whim. If you keep leveling up you'll be able to defend yourself and your family/friends much better, and that's without even touching the possible immortality, multiverse hopping or god-slaying you can participate in if you survive long enough.
You can treat gold as not the reward, it's just the thing you use to buy stuff that'll keep you alive to get the real good stuff.