The Mongols wiped out several indigenous languages, and the results of their conquests in places like Iran resembled the consequences of later European conquest in other places. They brought holocaust, to many. And famine, plague, etc.
Would you say the English did not add to the culture of India ? Railroads, say? In a couple hundred years, will the added wealth and so forth of introduced English technology not swamp the records of initial impoverishment ? Thus it was, with the Mongols - one of the few comparable empires.
In general, I think you underestimate the effects of disease and economics, and overestimate the effects of xenophobia.
The Western conquests are recent, and the technology (especially of language use) much changed. Time will tell, if the descendents of the Dutch and Portuguese in the colonial Indonesian Islands are still speaking Dutch and Portuguese in three hundred years. I notice few Swedish speakers in the territories conquered by Gustavus Adolphus. No one in Europe speaks Latin.
The major languages in places like Peru and Ecuador include Spanish, but only as one among others. English is more common than German, in China, and only via America, which never conquered the place.