The guy majored in physics and computer science. There's nothing wrong with compensating people who have proven they can scale things (organizations) commensurate to their record.
I find it difficult to believe that any sort of scaling of the org could have occurred in the span of six months thanks to the efforts of a single individual.
not saying value is right or wrong. but that is the present value of a stock award that vests over time. this is valuing his expected contribution and whatever it took to get him to join versus previous company / other opportunities. to be paid out over 4 years.
Presumably the management at Dropbox has no reason to throw away money. He received this on a vesting agreement and has to hit goals to receive it.
He has run several multi-billion dollar units at Microsoft. Likely he had many other opportunities on the table.
Fact is it takes more than code to build something that lasts. You need to know code, understand people that write code, but there is a lot more to it than code. This guy was hired to multiply the efforts of hundreds of coders -- so all he had to do is improve each of them 5% to justify his salary.