Colonialism, in the eyes of the great and the good, is responsible for all of the third world's ills.
This hypothesis is obviously absurd, but if you've ever tried to argue this with a progressive, it turns into a game of whack-a-mole. You point out that social indicators were better under colonialism, they claim that the fact that it got worse afterwards was actually due to the colonialism (how, we are never told - something about borders being too straight or something.).
This is, of course, an enormous game of shifting the goalposts. The only way to win is to pin them down about what the goalposts are ahead of time. Naturally, they will pick goalposts that they think are so narrow that you couldn't possible sneak in. Fortunately, as long as you know more about the history of a couple of what we economists call 'natural experiments', they probably won't pick small enough goalposts even under the most self-serving of definitions.
Shylock: Let's assume that colonialism might have some negative effects that survive after it leaves. Presumably these effects don't last forever. How long is it reasonable to use that as an excuse before you have to admit that colonialism can't be the real problem? In other words, if you have a third world country that was colonised by a European power and then gets independence, how long should it be before they're able to become a functional country?
Progressive Foil: (thinking quickly about time frame of African independence, trying to come up with a number greater than the maximum period of independence). Hmm, maybe 100 years. (Thinks again, adds a margin of error). Maybe 200.
SH: Haiti has been independent for almost 225 years, and it's one of the worst places on the planet. How does that work?
PF: (if uninformed) Um...derp...
(if a bit more informed): That's different! They were slaves brought in from all sorts of places with no cultural or linguistic links.
SH: I thought diversity was our strength.
PF: Plus the US Marines occupied it for 19 years in 1914.
SH: That's fair, it's possible that the place was just about to turn the corner after a mere 125 years of dysfunctional independence, I guess we'll never know. Odd that the US occupation was surprisingly functional compared with the rest of its history.
PF: It was not! It was horribly brutal and racist.
SH: I take it you haven't read much about the administration of Papa Doc Duvalier.
PF: (flicks through Wikipedia page) Hmm. Yeah, that's not ideal. But still, you can't make the comparison.
SH: Okay, okay, fair point. Haiti isn't a perfect example. Let's try a different thought experiment. African countries are inevitably marred by their colonial occupation. If we could see what Africa would look like today if it hadn't ever been colonised, it would be a lot more peaceful, rich and stable.
SH: Ethiopia was never colonised.
SH: Yes, and you may notice that it's not Switzerland.
PF: Okay, but it's going a lot better than its neighbours.
SH: See, at this point, I know you're just guessing. You know how I know that? Because I researched this in advance. Let's compare Ethiopia with two nearby neighbours that were colonised - Djibouti, which was colonised by the French, and Kenya, which was colonised by the British. Here's a few numbers.
Ethiopia: GDP Per Capita (Nominal) $575. Homicide Rate: 12.0 per 100,000. Life Expectancy: 64
Djibouti: GDP Per Capita (Nominal) $1692. Homicide Rate: 10.1 per 100,000. Life Expectancy: 61
Kenya: GDP Per Capita (Nominal) $1416. Homicide Rate: 6.4 per 100,000. Life Expectancy: 61.
PF: Ah ha! Their life expectancy is 3 years higher!
SH: Yes, I took a fair sampling of statistics, not just ones that support my case. But compared with its neighbours there's more murders, and they're literally one third as rich. You were the one claiming that Africa would be functional except for colonialism. A life expectancy of 64 puts it up there with paragons of civil society such as Yemen and Senegal. I'm even willing to grant you that it's broadly similar to its neighbours, but this doesn't exactly prove your case.
PF: Hmm, this is a puzzle. I'm sure I'm still right, but I need to research this more.
As Mencius Moldbug once said, I will win because I know all of his arguments and he knows none of mine.