https://www.reddit.com/r/space/comments/ld4vlq/gabbard_diagr...
Let's not do that again.
I suspect the diagonal line between the colors represents the duration of a stable orbit at that altitude. Anything with a more elliptical orbit will drag when it's in the closer phase and decay. Anything with a lower orbit is already slowing to a tighter orbit (and more drag on the denser atmosphere).
This would increase the "drag" and deorbit debris in higher orbit much faster. My physics intuition says it would affect smaller things much more than bigger, which is what you'd want.
If it can ever be practical, I don't know, but conceptually that works, right?
While it does look like 500km is about where things fall out quickly, it also looks like there's a sweet spot of altitude to orbital period ratio where stuff just stays there. Outside of that sweet spot stuff still falls out fairly quickly.
Edit: I think I now understand what you mean, but I suspect you're not realizing that every satellite appears twice on the chart: once in blue, for its periapsis (perigee, i.e. low point of its orbit), and once in red, for its apoapsis (apogee, i.e. high point of its orbit). You're probably noticing that some of the red dots, at high altitude, are decaying quickly, but that's because the corresponding blue dots are at low altitude.
Note also that orbital period is a monotonic function of altitude. More precisely, the square of orbital period goes with the cube of the semi-major axis (the long axis of the orbital ellipse).