What if Pol Pot had not invaded Vietnam, thus ensuring the downfall of his regime? What if he had just decided to cement the link with China, not intervene in foreign affairs, and focussed exclusively on his grand design for Cambodia?
At best Cambodia (or Kampuchea) if you prefer, is an agrarian state which is the poorest on Earth. The mortality rate is high and the country is now easy pickings for any of its neighbors. The Khmer Rouge's days are numbered how ever you may count it. In the end they were turning even on their own. At worst you have another nation like Somalia with rival factions fighting each other and outside forces (other nations) vying to pick up the pieces.