There is no reason for money supply to keep growing infinitely. 

Mining serves two purposes
a) fair distribution of money supply
c) protecting the network

Once there are sufficient coins "in the wild" the reason a goes away but reason b remains.

Miners are paid by fees.  The block reward is simply a subisdy.  Fees today are too low to support a robust network so the protocol provides a 50 BTC subsidy.  The declining subsidy over years provides sufficient time for network to move to be supported completely by fees.