At risk of falling victim to Poe's Law, I'll reply:
You're supposed to have a maximum of 6-8 Medium fights in an adventuring day. Even in the DMG it never says every day has to be an adventuring day. It even has a whole section on downtime between adventuring days. Even if you run non-sandbox and therefore want to use the "encounter balancing" rules, there are absolutely no guarantees that there are multiple dragons attacking your town every day. Even on adventuring days when something happens, it's as possible for your group of 4 20th level PCs to wind up fighting a hobgoblin battalion (500 hobgoblins, 200K XP) or 71 Wights (199K XP) as 2 Ancient Blue Dragons (150K XP).
So no, you won't be richer than Smaug in a week.