When D-3 dips really low, i think it gets peoples' attention and they may tend to fill the World back up.
For example, I had no intention of joining a 4th World, but when i noticed Phelan D-3 dropped to 47 or so back 7 seasons ago, i joined cuz with fewer teams i thought i could compete for my 1st Final Four or beyond. I never got to the Final Four but i noticed last year that Phelan D-3 popped back up into the 50's or 60's, and at this point i qualified for D-1 (which is my favorite division), so i moved up.