@2centsdo you think that erasing the boundaries created post WWI and allowing the formation of nation-states like the millets in the Ottoman Era (after the defeat of ISIS) would help the region gain stability?
In a way, this is what appears to be happening now, in a rather brutal and violent fashion. But I don't think it holds the promise of peace and stability ahead. With the exception maybe of the Maronites, the religious communities were and are too geographically dispersed to form nation-states on the model of Europe. And the Maronites, when they had their opportunity, grabbed too much non-Maronite territory to form Lebanon, with disastrous consequences.
When people are asked to provide an alternative arrangement to the partitions that actually occurred, there are no answers that can be given which you can say would definitely have provided order of the kind that the Ottoman system successfully provided for centuries, because the realities of nationalism had changed the rules of the game to the extent that violence, ethnic cleansing and partition were inevitable. We can be thankful that none of them were carried through with the same ruthlessness and on the same scale as what Europe experienced 1914 - 1945, although in some ways what's happening today can be seen as the same logic working itself out.
People generally offer three alternative arrangements to what actually occurred:
1. "The new states should have reflected the ethnic and sectarian realities of the region." The problem with this is that (a) the region had never been governed along such lines, and (b) introducing this principle in 1918 would have resulted in the creation of literally dozens of micro-states, all even greater prey to the logic of divide and rule, but only after many forced population transfers (ethnic cleansing) had taken place. This argument also implicitly suggests that the creation of states in which a common citizenship rather than ethnic or religious affiliation would be the defining factor of an individual's identity was impossible, and therefore it retrospectively justifies the continuation of pre-modern forms of communal identity as the 'natural' way of things in the Middle East. In other words, it embraces the same kind of Orientalist way of thinking, of an unchanging region defined by religion, which the very people making this argument often decry.
2. "The British should have handed the entire region over to the 'the Arabs'." The problem with this argument is that 'the Arabs' didn't exist as a unified political actor. The British had alliances with a number of different Arab tribal leaders in the Arabian Peninsula, most of whom were at each others' throats. The Arab populations of Syria and Iraq largely remained loyal to the Ottoman Empire throughout the war, and in its aftermath went at each others' throats over how to divide the limited spoils granted them by the British. The result would likely have been a bloody, immediate civil war in which all kinds of ethnic cleansing and massacres would have shaped whatever kind of state of states eventually emerged.
3. "The British and French had no right to be there in the first place, and therefore the region should have remained under the Ottoman Empire." This argument is based on the idea that everybody should just leave everybody else alone all the time. Putting aside the fact that humans could never progress that way, it's just a completely unrealistic way to view these things. The Ottomans themselves were foreigners who had conquered the region by force and imposed their rule on its population. The British and French therefore broke no moral code by doing the same. The Ottomans had been happy the use the interference of Britain and France throughout the 19th c. in order to keep the Russians from conquering them once and for all, and then made the decision to enter WW1 with the partial aim of reconquering territories lost to them over the previous half century or so. Unlucky for them, they chose the losing side, and so paid the price. In any case, Ataturk had no interest whatsoever in retaining the Arab lands for Turkey, and wisely let the Europeans have them as he went about constructing his modern state.
Basically, my argument is that nationalism is to 'blame' (if we have to assign blame - personally I see little point in moralizing back across the decades and centuries), but that it's hard to envision how it could have been significantly different given that nationalism was destined to shape the postwar world order.