Look at it this way. Would we allow a president to decide who will cast votes for the next election? It's a legitimate question whether we should allow popes to continue to do so, especially since the Church appears entrenched in a downward spiral. The US allows the people to select electors for their president and vice-president, and they cease to be electors upon the fulfillment of their office. 
There is no reason why the people who elect the next pope have to be cardinals. There's nothing theological or magical about having 120 handpicked electors.