Unless it fails, of course.

At this stage it cannot "fail" in the traditional sense. Even if it enters a downward spiral, at any point in time it could be sold for the user base alone (and the price would still likely to be bigger than Heroku)

That is a very dangerous attitude. The moment you think you are too big to fail you are more at risk than at any stage before then.

Dropbox could still fail overnight. It would have to be a bad set of circumstances but they're definitely not out of the woods yet.

Remember Blockbuster?

Blockbuster has nothing to do with this. That was a failure in the traditional sense. I never argued that somebody is "too big to fail". The parent poster argued that if Dropbox failed, it would not be a big homerun. What I argued is that even if Dropbox valuation failed by say, 90% and it got sold, it would still net around 500MM. That would be a failure for the latest investors, but definitely still a heck of a success for YC (I am disregarding purposedly here liquidation preferences, etc, but you get the idea)