First of all sorry for my poor english...
Well as the title says, I would like to see implemented such a function.
It's up to you to decide the exchange rate of course, maybe you could also implement a dynamic exchange rate like there is in GW2, but the possibility to exchange my in-game currency into gems would be realy appreciated. In this way a paying customer would still have an advantage over others but at least everyone can spend their gold for something else other than just upgrades.

It is a nice suggetion. Don't know if they ever are going back to the beginning.

But you should know the history when the game started you could buy everything with gold and with crystals.

By the jun 2013 you could only buy lvl 1 gear, upgrade and buy a clan for 10000 gold

By sep 2013 you could only buy upgrades and clan for 10000 gold

By nov 2013 you could only buy upgrades and clan for 1000 gold. (1000 gold for a clan is to low in my oppinion)

Don't know what the next step will be.