In any case, the $X in S&P 500 at t=0 versus the IRR of a venture fund is not an apples-to-apples comparison. There are many ways to meet your capital calls, and I suspect that sophisticated investors aren’t keeping the cash in their checking account waiting for an email.
If I have a lump of cash I need to deploy, I can buy lump sum SPX at t=0. I can't call a VC and say "here's X money, invest it all immediately."
The fairest comparison probably would be lump sum SPX against a blend of VC IRR and money market, converted from one to the other at a typical capital call rate, but as the number of variables increases so do the number of assumptions, and given the low returns on money market funds I don't see how the extra complexity adds much to the story.
Am I missing something?
If you've got 50% committed, then you need to do even more careful cashflow planning.