There's some gross oversimplification in your reasoning. Just to cite a few.

Egypt? Incredibly wealthy under the British, not so much under self-governance.

You seem to forget that Egypt has a history of more than thousands years and was a superpower long before the British empire even existed. And after that, there was Persian, Greek, Roman, Arab and Ottoman occupation. Thus comparing the country's wealth during and just after the British colonialism is reductive. How about trying to do the same comparison throughout these periods and see under which one Egypt was at its peak?