Originally Posted by Unregistered

This is survivorship bias at it's finest. You are occupying one of the spots that 9 other people were theoretically vying for as A1, but quit along the way.

Even if everyone wanted to "stick it out", the firm would still have needed some way of driving 90% of every cohort to quit. There is no business case to make up 100% of each batch as partners. Only 1 in 10.

B4 can count on natural attrition to do the job of filtering for them because the working conditions and pay at the junior ranks is so terrible that the vast majority quit.

In other industries where the high pay makes it far better upside for juniors to stick it out as long as they can, such as law firms, consulting firms and banks, they employ an "up or out" model - if you don't make partner within X years, you are kicked out of the firm. Go read up about the "Cravath System".

If B4 ppl didn't quit in droves on their own accord, you can be very sure they will be employing the up-or-out system. In short, your logic is flawed.