What about holding the additional $70k per startup in escrow and dividing it among those startups that deserve to stick around?
Basically, if you take 60 startups, give each $80k and put the $4.2 million (60 * $70k) in the bank. When that batch of YC startups gets to $20k left in the bank on average, figure out which ones show promise and which ones should be deadpooled. Assuming a 2/3 deadpool rate, that leaves 20 startups to divide the $4.2k among, or approximately $210k in convertible debt per startup to reach the next milestone.
This is more inline with your essay about the future of funding, in which you speculate that funding will become more continuous.