Because Palem your suggestion doesn't address the issue. What will happen is for one age you will have kds with decent numbers - then as ages progress kds (ghettos) will have less and less players - until they are back to the ratios we have here (as we saw on genesis). So what then? cut kd numbers again and things are ok for an age and it repeats until it is 1 player is a kd. Perhaps with automatic attacks too. No thanks.
Also, the fact that you have to drop players is a huge issue. Kicking people out of kingdoms, imo, cannot be the sort of mechanic we need to encourage people to play.
It is, at best, a short-term measure that completely bypasses the real issue - we need more players. At worst it is a mechanic that is doomed from the start to fail and one that might actually cause players to quit.