> I believe one of the worst outcomes of colonization is that these countries lost their monarchies and lacked a natural progression to diplomacy
I think this is a very loaded and fraught perspective that ignores a large amount of context.
Colonial powers generally co-opt local heirarchies and utilise the pre-existing state machinery to expediate the process of resource extraction and pacification since doing it from scratch is usually too costly and prone to instability. In many cases this may inflame pre-existing class confict and further entrench social division. It is rarely the case that pre-colonial power structures simply just vanish and are replaced by the colonial force, and furthermore they don't simply vanish post-colonisation leaving behind a template-less society.
Ex-colonies don't exist in a vacuum. Every society on the face of the earth is embedded in a complex global web of economic and political influence. Post-colonial nations can still be implicitly, and sometimes covertly, subjugated through asymmetric trade agreements, power projection, and a whole range of other processes. It is naive to assign blame of a corrupt or floundering region to a simple moral decay in an isolated system of people that just never figured out how to govern themselves. The answer is found when one instead considers the given region's place in the continuum of economic and historic processes that are far too complex to simplify into an narrative independent of context.
And finally, how does a monarchy naturally progress to "diplomacy"? To my knowledge there doesn't exists a single theory that can describe a universal archetype of a how a given human society is supposed to "naturally" develop. Even in western countries, the process of economic development from a feudal mode of production to the current capitalist parliamentary-democracy, is an extremely complicated and poorly understood topic that covers an area of research far larger than the scope any single historian or political theorist. Everything we see indicates that there is absolutely no fixed model of social development, especially not one that isn't contigent on an unimaginable number of nonlinear factors. I think it's reasonable to say that the development of any given society is completely unique to itself, and is the result of it's own unique position in relation to the outside world, and to history.