That $15 mil investment might be acquihired, or continue to be unexcitingly profitable, or put up on the chopping block for its patent portfolio. They might not even lose money, but.
Also keep in mind that VC funds as an asset class are historically a bad investment: http://www.slate.com/blogs/moneybox/2012/05/07/most_venture_... http://avc.com/2013/02/venture-capital-returns/
It's not just that the VC expects the average startup to fail; it's also expected that the average VC fails - or at least underperforms the S&P500.