Trading post to convert resources

I have often been frustrated at times when one resource is maxed out, yet all upgrades for that resource are maxed out as well, thus leaving me with a full storage tank, and nowhere to spend it. At Town hall 9, one can choose to upgrade walls with either elixir OR gold. Why isn't this option available for other upgrades? Also, there should be a "Trading Post" building that can be obtained at a certain level - let's say level 9 TH. This trading post would allow players to "buy" other resources at a certain exchange rate. Say I have WAY too much elixir, but I need gold to upgrade an archer tower. Why not give players the ability to buy the gold they need with their excess elixir at an exchange rate of 2:1 or 3:1?

This idea is ruled out, it would have negative effects on the game economy.