That's a fair point, but cliffs are more severe than exercise windows. Many employees still get to keep some of their equity in a 90 day exit window scenario. I think Kupor modeled it at about a third of vested shares on average.[1]

More mature startups may be able to simply abolish the long-term incentive that the 90 day window provides, but I suspect younger (<30 employee) startups need added turnover protection. Perhaps backloaded vesting would be a more comparable replacement? Transparent, predictable, fair, and not as harsh as cliffs. You keep what you vest, but 70% of it vests in years 3-4.

[1] (Although there's certainly unfair variability based on personal financial circumstances in that average.)