Why does the ghost of Christmas Present say when talking of Tiny Tim If he be like to die then he had better do it and decease the population?

This follows Scrooges query when they stood in the Cratchit household on Christmas Morning. Scrooge sees Tiny Tim and asks of his further, the ghost states I see an empty chair by the fire and a crutch without an owner. Scrooge is horrified to realise that Tim is destined to die. The Ghost rebukes Scrooges plea to say that is not so by saying "If these shadows remain unaltered by the Future, none other of my race,'' returned the Ghost, will find him here. What then? If he be like to die, he had better do it, and decrease the surplus population.''