As in modifying the code on an agreement with all parties involved, is there a chance people might want more than 21 million Bitcoins?
Then the value of BTC will drop and nobody wants that. Food thing about BTC is that you know exact number that would be mined if you will go by the principle " Ok we need more coins, lets vote and make more " then there is no difference with a country thats printing out money on a daily basis.