This only works if, in addition to all of the other possible risks we all face in life, this CS grad could simply disappear because a CS student in the new freshman class is more interesting.
Snapchat is risky because it's audience is as fickle as they come.
In either case the point was to illustrate that, all risks considered, snapchat is still worth a lot of money. I have yet to see anyone (HN commenters, tech press) argue about what their valuation should be, which seems like the reasonable follow up to "It should not be $3 billion".
We can agree that its not zero, then what basis can we use to agree that $3 billion is inaccurate?