While I think having some sort of renewable-ish form of gold collection would be fun as a sort of cheat object, I get (I think I get) why it's done the way that it is. Purely to keep us digging, to keep us expanding our kingdom area constantly digging to get more gold, silver, titanium, obsidian etc, most of which can be turned into gold if desired. Titanium sells for a fair amount of gold, I think it's 125 gold for 10 titanium, although it seems that we don't really come across titanium all that often, or I haven't anyway.
What I would like to see is maybe the ability to hold down shift while clicking the sell or buy buttons to sell 5 times the amount shown on the screen, so for example hold shift and click sell button to sell titanium and I would sell 50 titanium for 750 gold per click. Maybe even hold down Ctrl and click to sell 10 times the amount shown at a time for when selling things like wood or stone.