Wouldn't this have the effect of changing the risk/return balance? For those joining your company early on, the risk would remain the same, but the return would fall sharply (by ~50%), while for those joining late in the game, the risk would remain the same, but the returns would increase a lot.
If everything else remains the same, people would be less willing to take risks and join early stage companies, instead trying to join near-IPO ones, where you can get a disproportional payout from minimum risk.
To maintain the same risk/return profile, you'd need to pay much higher fixed salaries to early employees and lower to late employees, which would probably drive the startup bankrupt on the early stage page.