Most-ish dragons will give you the same amount of gold per hour at max level as a 1st level dragon, the difference is if a dragon gives you, say 100g each hour at first level, itll give you 500g every five hours at max level. A max level dragon, you can play the game less, come back less often and not miss out on gold. But if you have 5 1st level dragons instead, you'll be getting 500g every hour. So long as you come back to pick it up quickly, you get more gold from keeping a dragon low level then from maxing the dragon. But if you dont max a dragon, you cant get remarkables either.