The only argument in favor is that large numbers of recent graduates will vote, entirely selfishly, for someone who promises to alleviate their debt. Or, as we could also put the same idea, Sanders wants to buy their votes with $1.6 trillion of our money. Again, this is a private benefit to Sanders at the public cost to the rest of us.
At least that does show us that Sanders is not some new and different sort of politician. After all, politics is the art of buying sufficient votes with other people’s money, isn’t it? It appears that he’s pretty good at that.