The only thing wrong with the Domino Theory is that except for Laos and Cambodia, the dominoes didn't fall in Southeast Asia, but elsewhere. Within a year of the fall of Saigon, Angola, Mozambique and Guinea-Bissau went Communist, as did Grenada and Nicaragua in 1979. In 1983, Surinam entered the Soviet bloc for a short sojourn--the high tide of Soviet imperialism--before things started to go sour for the Evil Empire.
The argument on the dominoes is how much had been altered and changed from 1950 to 1975 in the world/Communist situation in Asia.