The company's valuation at the last financing round already prices in the high likelihood of failure. That is, on average, the VC industry does a good job at as

The company's valuation at the last financing round already prices in the high likelihood of failure. That is, on average, the VC industry does a good job at assigning valuations to companies (see published reports on average IRR of the VC industry). As you point out, there's huge variance in outcome which may affect your appetite for shares, but when it comes to assigning an expected value to your shares, the last financing round is an appropriate place to start.