The concept of "natural countries" as applied to most of the colonies is an anachronism. The nation state was a European construct, formed as a result of the Treaty of Westphalia. Africa and much of the Middle East (and parts of Asia) never had a system of organizing around national identity, only around tribal or religious identity. Therefore, the colonial method, judging by the standards of the time, was just as valid as any other--that is, unless each tribe was to be awarded its own country, which of course was impracticable then, and is impracticable now. Iraq may not make much sense, but combining Iran and southern Iraq just because of a shared religion doesn't make much sense, either.
In short, even today, the concept of the "nation state" is still alien to much of the world, which is why instability still exists there. The instability will continue to exist until war forces groups into a common identity, or eliminates them.
I agree. However don't you think Sudan could have been done better? The Arab Muslim north vs the Black Christian south, that was never going to work in the long term. And they ended up splitting anyway.
As for the Indian partition, maybe they could have been forgiven for not realizing how powerful the religious identity was to a large segment of the population.
Iraq is a tricky one, with the three groups (Sunni/Shia/Kurdish) all seemingly unwilling to share a common national identity.
When I talk about "natural nation states", what I mean is that they are a successor to something. The Chinese nation-state for example, is merely a modern incarnation of the Chinese civilization, which has had a central authority (very much like a modern nation state - even including a national bureaucracy) for over 2000 years. There have been many civil wars over history, in which the state has been split into various parts, but over the thousands of years, the majority of that time has been spent under a central authority (Tianzi - The Emperor of China).
The vast majority of the Chinese population shares a very similar cultural/ethnic/linguistic/religious identity, that of being Han Chinese. Whereas minorities like Uyghurs and Tibetans combined only make up 1% of the overall population, there is no significant demographic challenge like there was in pre-Partition India, or modern day Iraq, where there is no clear majority... and the various conflicting demographic segments almost equal each other in numbers.
When America was a developing country, they had a clear and dominant ethnic majority, so did Britain (in fact Britain still has that today).
After becoming developed countries, ethnic identities gave way to "national identities", for example if you had a poll in America right now, probably 99% would identify themselves as "American" - which is the glue that binds them together.
When it comes to Iraq and Syria though (developing countries), they do not have much confidence in a common national identity, preferring to align by religious sect. ISIS fighters come from all over the world, it is not the nation-state that binds them, but rather a common religious motivation.
For China it was easy to transition to Chinese nationalism, it seems like a continuation of past history and culture, just changing the vessel from a civilization to a nation-state. But for countries like Sudan, these people were forced together into a country by colonialists, seemingly using a ruler to draw right-angles through them. It seems very unstable.
Just my personal opinion.