It's a real problem. I saw people boomerang back to Google after 18 months at one or even two levels higher, much faster than they could have been promoted internally.
The 85%ile of L used to be above that of the lowest 15%ile of L+1 so in the year that you demonstrate L+1 you'll get compensated as L+1.
Now, the 85%ile of L is less than that of the lowest 15%ile of L+1 so until you actually get promoted there's no real compensation difference. Which makes leaving for L+1 so much more attractive (and also going above-and-beyond less attractive).
The 85%ile of L used to be above that of the lowest 15%ile of L+1 so in the year that you demonstrate L+1 you'll get compensated as L+1.
Now, the 85%ile of L is less than that of the lowest 15%ile of L+1 so until you actually get promoted there's no real compensation difference. Which makes leaving for L+1 so much more attractive (and also going above-and-beyond less attractive).