This is why mining will become useless within a few years the price for equipment will be too much.
The market will adjust as necessary and mining will never be useless cause it keeps the network alive.And even then when the market eventually approaches saturation (you'll never keep getting the same efficiency increases) the difficulty will retarget accordingly to maintain the ~ 10 minutes between each block. Hence your difficulty will eventually slow down in its increases and may even fall. There is also always an incentive to mine - even when coin generation stops they will (hopefully) be paid in enough fees to have a reasonably large network.
<--kuddos to said user. 
