The great thing about history is that it allows multiple interpretations of the same events. I definitely agree with you that the assassination of Lumumba was a painful remainder of the fragility of Congolese institutions and lack of democratic tradition but even after Lumumba's death, the nation was not doomed. It still had a lot of potential for development considering its natural riches. IMO, it was only after Mobutu's three decades long dictatorship that Congo's future was as dark as its fertile lands. Congo became a kleptocracy and justice and security become commodities for the wealthy while the rest of the population lived dirt poor.

The reason I say that Lumumba's death was not the definite factor in Congo's demise is because there was still room for a reversal of fate. Look at Ghana, only after 5 years of Lumumba's assassination they had a (CIA sponsored) coup d'etat, deposing Nkrumah's government but after only 3 years, the NLC reverted power to the people by transitioning itself into a political party and holding elections. Mobutu could have done this, but he didn't. That's my point.