C.P. Sarvan's response to Achebe's criticisms consists basically of the assertion that we cannot identify Conrad's point of view strictly with that of his narrator. Marlow, in other words, may be a racist in his descriptions of Africans vs. Europeans in Heart of Darkness, but this doesn't mean Conrad endorses those views. And this is, to a degree, persuasive, since we have no reason to believe that Marlow is an absolutely reliable narrator.
Nevertheless, Sarvan can take his argument only so far. Since 1980 much in literature and in the general culture has changed. It is difficult, in my view, to make the case that Conrad's descriptions, and the attitudes that underlie Heart of Darkness, are not racist in some degree. This may not have been recognized in earlier decades because racially-oriented thinking was so endemic in Western culture that it did not occur to many people to question these attitudes, especially when there was already an assumption that an author was liberal or progressive. Conrad is clearly critiquing the imperialist system. But many earlier readers probably did not realize that most of Conrad's disagreement with imperialism lies in the negative effect it has on the colonizer rather than the colonized. It is somewhat similar to Thomas Jefferson's critique of slavery in his Notes on the State of Virginia, where Jefferson seems more concerned with the fact that slavery corrupts the white man than with its effect on the enslaved person. For Conrad the fundamental tragedy seems to be that Kurtz, in his megalomania and psychosis, has destroyed himself. Conrad recognizes that the exploitation of the colonized people is immoral and destructive, but this does not stop him from portraying "the natives" in a stereotypical and demeaning way. And it's significant that the revelations about Kurtz are presented ambiguously. We are not shown exactly what has happened and why: the details of Kurtz's having gone off the deep end are hazy and mysterious, analogous in some way to Conrad's dense, often mystifying prose. It's as if one expects the whole story to be revealed in the end as an opium dream, a tale too lurid to be believed.
Conrad's status in literature has been so iconic that, in conjunction with the entrenched nature of Eurocentric attitudes, these facts were glossed over or unnoticed. The same has been true of George Orwell's writings about colonialism. Orwell was left-wing, and he was bitterly critical of the colonial British system--therefore people concluded that his views could not possibly have been racist in some degree. Like Conrad, his focus is chiefly on the uncomfortable results of imperialism upon the Europeans themselves. This is not to say that Conrad or Orwell were not great writers or that their works should be "excluded from the canon," though in the main, Achebe's criticisms of Heart of Darkness are valid. Our general perceptions of both Conrad and Orwell have been fashioned mainly by British and American commentators. Much in literature needs to be reevaluated from wider points of view, though as with anything else, a recognition of the time and place in which it originated has to be taken into account when judging it.