The problem isn't with early VC money. Dropbox's early backers would have still made a gigantic return on Dropbox setting at a $1bn valuation.
Really, it's not even a problem with their series B ($250M at $4bn). $4bn is a reasonable eventual valuation for Dropbox.
The problem is that they didn't stop there. They raised another $850M in 2014 (in two rounds), which was basically sold on the promise of it being necessary to "win" the storage wars and acquire a monopoly.
By 2014, Dropbox was already a stable company with hundreds of millions in revenue. The only reason to raise money was to shoot for hitting Google/Facebook status (owning a lucrative monopoly). If they had instead accepted that their eventual outcome was a $3-5bn software company, everyone could have won: employees, investors, founders, etc. (Atlassian is an example of a company which did eventually raise substantial VC, but instead of gunning for a monopoly IPOed at a $4bn valuation.)
The problem is that founders ultimately have huge egos and want to be the next Mark Zuckerberg or Larry Page: commanding monopolies so lucrative that they can spend billions on zero-return "cool nerd shit."