You're going to miss startups that might have succeeded if you had invested this way. You take a percentage of the companies you invest in because you make it more likely for them to succeed.

Given the funnel of startup failure, there are probably more startups that didn't succeed because you passed than startups that succeed despite your passing.

But what metric can track might-have-beens?

My point is that only looking at those that are successful is missing most of the data on people they should have funded. Assessing this is really hard, but they shouldn't fool themselves into thinking they have all the data.

Your insight is correct, but doesn't seem to be actionable in any way.

Yes it is: reflect on the interview process, not just on the rejected eventual successes.