Moreover, you are addressing a question never asked, and that is somewhat irrelevant, which is the question of whether the local populations were better after colonialism than before. I am not going to address that question.
The important strategic point is that everywhere that colonialism has existed, any regime where an outside power dominated a local population, however underdeveloped the population before hand, sooner or later the local population, having become better educated and more developed, has sought to cast off that yoke and cease to bear oppressive burdens from their overlords. Read the Declaration of Independence.
That in the 20th century communists sought to exploit anti-colonial "sentiment" to spread communism, does not justify trying to preserve colonialism, or make the local opposition to colonialism less valid. That communists have been able to exploit the existence of oppressive local leadership does not somehow justify that oppressive leadership.
The point of effective counterinsurgency is to be able to provide a better alternative than the communists provide. When the communists at least promise a better life than the miserable one we were fighting to preserve, then loss is inevitable. Getting on the right side of heaven is the key idea of Sun Tsu. American idealism is the belief that the ideas expressed in the Declaration of Independence are universal, not just applicable to some guys who didn't like a tea-tax.
Actually, they left the third largest economy (by GDP) in the western Hemisphere. It also helped, however, that unlike the Brits, the Portuguese sent only men, who bred with the natives and then the Africans (mass European immigration to Brazil was a late 19th/early 20th century phenomenon).
Angola and Mozambique were different in that they were discovered later, and the Salazar government didn't start sending emigrants over in large numbers until the 1950s, at which time colonialism was on the wane.
Someone who brings up the past to make a point has to address the whole past, not leave out the bits that challenge his thesis. Europeans left their colonies in a hurry because some ambitious native leaders fought expensive (in men and materiel) revolts against them and others threatened to do so. Europeans owed no more to the natives than native empire-builders did.