That is a pretty big assumption, and I would like to highlight it more.

The reason why it's a reasonable assumption is that in any material outcome, common share preferences converge to preferred share preferences.

Yes, once in a while you have an exit at 0.3x to 0.8x latest round where the investors are made whole and common share holders take an excessive haircut or are wiped out entirely. Good Technologies is an example here.

But looking at VC returns, almost all come from ~5x+ exits: "home runs". In fact, most VCs will block acquisitions at less than 2x latest round valuation except in case of corporate distress.