Even if there had been no World War I and World War II, we would have to assume there would still be more isolated conflicts between the empires. The nature of empires is to expand and to control resources. At the beginning of World War I there were six empires in Europe, all competing for similar goals and territory. Sooner or later, some of them would have fought.
The demise of imperialism, which was well underway at the end of World War II and more or less finished by the early 1960s, may have happened a bit later without the world conflicts and their aftermaths to spur it on, but it would have happened. Imperialism as a system was not killed by the wars themselves, but by human progress and a worldwide movement for independence of nation states and peoples.
I think that imperialism is now an idea whose time has gone. I think that ideas like these will die off eventually, one way or the other. I think that ideas about justice would have evolved to the point where countries would no longer have large empires even if these wars had not happened.
I think that you can see an example of these changing attitudes in how the US dealt with the Philippines. As soon as we took over that country, we promised to give them independence eventually. Once WWII was over, we did as we promised.
I think that people would gradually have come to the idea that imperialism (like slavery before it) was wrong and it would have died out on its own.
I would probably say yes, because there were myriad lessons learned by imperialist powers in the various places they pulled into their empires that you might think would act as a deterrent to imperialism long before WWI and WWII and it didn't stop anybody.
The Roman Empire fell apart thanks to over-expansion, among other things, and various other empires struggled with administration and defense ove vast swathes of territory, Alexander, the Mongols, various of the Chinese Empires, etc. How many empires alone were brought down by attempting to subdue the area we call Afghanistan?
Yet powerful and wealthy people and nations were still chomping at the bit when it came time that they reached a point where they could expand and repeat some of the behaviors of the earlier empires.
So I think those two wars had little effect and their non-existence would also have done little to stop the imperialist tendencies of rich and powerful nations.
Any answer to a question will only be a guess. There is no objective way of answering question like these. My personal belief is that there were many forces, that originated much before before World War I, led to demise of large scale imperialism all over the world. World War II did play an important role in speeding up this process, but it was definitely not the main force behind it. Speaking of World War I, to me it appears that it contributed nothing to the cause of imperialism, except causing the World War II.
The forces that actually led to decline of imperialism can be broadly classified in types of developments.
- Rise in feeling of freedom an equality of people. This is reflected in different movements across the world including American Revolution, French Revolution, Russian Revolution and Indian Struggle for Independence.
- Development of feeling of nationalism, which united people of a geographic region, and infused them with desire for independent nation.
- Developments in economic system, particularly the rise of mass production, which reduced the pay-off from exploitation of the people in the colonies. The logic of imperialism which assumed a fixed level of wealth in the world and believed in exploiting the colonies for amassing greater wealth, was no longer valid in the era of mass-production. The economy of mass-production required development of big and prosperous markets, which is possible only when the buyers of the mass produced items also become more prosperous.
Under the influence of these forces wide scale imperialism would declined substantially by the end of twentieth century, even if World War I and II had not taken place.