Cambodia is an exception to the rule, and in this case it's sadly true that Pol Pot "won". He did reach his goal.
By exterminating 20 percent of the population (the educated, the wealthy, the artists), Pol Pot and the KR did in fact remove all traces of Western influence from Cambodian life, and did in fact convert Cambodia into an agrarian, rice-exporting economy.
That specific goal was articulated by Khieu Samphan 20 years earlier in his Sorbonne doctoral thesis (BTW, all of the KR leaders were educated and learned Communism in France). They believed that Cambodia would become the true Marxist utopia by relying exclusively on its rice exports, expelling all "imperialist" thinking, and by turning the population into the two-legged equivalent of water buffalo (which was why thinking people - the "new people" - were executed).
So Pol Pot really did succeed, and it is evidenced in the devastation of today's Cambodia, as well as the shortage of teachers, doctors, lawyers, artists, and political leaders (actually these go hand-in-hand).
Ironically for the KR, it was after that first goal had been reached that they overreached for their second goal, which was their hallucinatory vision of removing the Vietnamese from what Cambodians call "Kampuchea Krom" - Little Cambodia - which is nearly the whole of South Vietnam (and had been Cambodian terrority until the 15th century).
So after a period of barbaric slaughters in the bordering Tay Ninh Province, and a fews days of rocket attacks on the town of Chau Doc, the Vietnamese finally responded. Phnom Penh went down in a matter of days, the KR was kicked into the jungle, and the Vietnamese occupied Cambodia for the next 12 years. But at least the slaughter stopped.