Eh, I'd argue Dropbox becoming popular is ancillary to the comment being an amusing example of a negative HN comment for a few reasons:

1. The comment categorically dismisses the entire product idea

2. As evidence for (1), the comment cites a rather complicated method that would arguably be out of reach for most users

3. The comment describes the method from (2) as "trivial"

So even if Dropbox had failed, the comment would still be entertaining, IMO.