What about the United States who have assimilated people pretty fast and of a variety of different groups? United States shows us a great example of people becoming more accepted when they become "Americanized" enough to be considered acceptable to the general American population. At one time some Italians were not even considered white by some people. Many even considered Irish as not far off from Africans on the social hierarchy at one point. If their standing could change why can't that not happen in other places.
You do realize that up until the 1970s America was pretty much apartheid, now granted that might not be bad if you come from some place worse off, but It wasnt the most free place on the planet. Even today thee is still very much ingrained racism.
It also can take 40-80 years to americanize, and even then, you still have to get passed the inherent racism of both sides of the coin.
Last point on the topic . Its a nation of immigrants, ask the people who were here before 1492 first how colonialism and then americanizing panned out, or those who chose to not do so.
Now to your topic..
The colonial empires were built on exploitation, racism and dominance.
Look at the end of Ww2, sovereign territory was just willy nilly carved up and passed around for smokes and a drink, treaties were made that were so lopsided that harpers off-season MLB contract is going to look like the deal of a lifetime.
Also Roma was about building an empire in an age and area not quite like today. Rome Romanizsd when if at all possible, and also persecuted, subjugated, enslaved, pillaged and mkre those who didn't render unto caeser that which caeser felt was his to have rendered.
I agree some territories eventually have joined there Stockholm oppressors, others have not and Others went a polar 180.
I will say that if no world wars, there would have been less inclination to diavest colonies and that could be either good or bad for local populations. But imagine a Belgian Congo 100 years later.