To play devil's advocate a bit here, the companies that are being devalued (on paper) are by definition the so-called "unicorns", or the outliers that VCs depend on to offset losses to their duds. So if you start knocking off double-digit percentages from a few of these outliers, pretty quickly your portfolio takes a huge hit.
Hypothetical situation with some seriously made-up numbers:
* 2x $100k for 5% investment in companies at say a diluted stake of 2% goes from $10B to 5B
* 5x $100k for 5% investment in companies who go bust
* 3x $100k for 5% investment in companies who are acquired, make 10% returns ($110k per).
Total investment: $1m
Return from non-outliers: (5 x $0) + (3 x $110k) => $330k for a net of -$570k
Return from outliers before devaluation (on paper): 2 x .02 x $10B => $400M
Return from outliers after devaluation (on paper): 2 x .02 x $5B => $200M
At this point, the losses/returns of the non-outliers are in the noise. But man, it hurts to have to explain the "loss" of your upside by such an enormous amount. I don't think VCs would shrug it off as just a part of their overall portfolio, since these are the valuations that prop up the overall performance.