I upvoted it because I agree with "The problem with your line of thinking is that open source [usually] doesn't pay. The best people in certain fields, meanwhile, only work for money, and occasionally only for large gobs of money", among other reasons.

I realize Dropbox might be able to open source their client without significant risk, but why take any unnecessary risks at this point? There's no reason to. Or maybe Dropbox feels that their source code's quality is nasty, and don't want to put in the effort to clean it up before open sourcing it.

It's solely the decision of Dropbox whether the Dropbox client is open sourced, and they should not be blamed for deciding against it.