It must be possible for Western civilization to be advanced by people of non-European lineage. After all, the English and the French are both just descendents of the northern European barbarians who painted themselves blue and sacrificed people to oak trees when Athens was really going. As I understand it, Aristotle expressed considerable doubt that such people were naturally capable of anything other than manual labour. He would no doubt have found peasants from 4th century B.C.E. Guangdong a relatively better bet.
To the extent Western civilization is particularly valuable, it is valuable because it discovered the combination of scientific inquiry, free expression, security of property and freedom of contract that appears to be the best way of organizing any modern society. There are more specific conceptions of Western civilization than that, of course, but a liberal order can accommodate them within civil society. The Platonic doctrine of the forms and the sing-a-long Messiah don't need state support. Indeed, the healthiest "Western" culture is the Western culture that has been forced off welfare.
As long as our interactions are peaceful and respect the rule of law and property rights, we can expect them to make us all better off no matter how diverse we become. In fact, the more diverse we are, the greater the gains from trade. And, so long as we preserve freedom of expression and religion, if our descendents end up believing something stupid and heretical, that's their problem.
All this only follows so long as our interactions are peaceful and newcomers accept the underlying rules. We can't just assume that this will always be so. Somewhere in human nature, there appears to be a tendency towards ethnic animosity. It would be nice to think this is just the legacy of history, and will decline as people become more exposed to egalitarian ideas and rhetoric, but this may be too optimistic. I found striking the outbreak of violence between Sikh and Filipino teenagers in British Columbia high schools, because no "legacy of history" can plausibly be invoked to explain it.
Now Canada, by its nature, cannot escape the difficulty of accommodating ethnic diversity. It may be that dozens of potential fault lines are more stable than just one or two. But we have to recognize that we are performing an uncontrolled experiment, and we don't know what the results will be.
Quantity matters. Canada's civil society clearly has substantial reserves of assimilative capacity, notwithstanding the state policy against it. By the third generation, we resemble each other pretty closely. But it can't be unreasonable, or racist, to talk about how far these reserves can be pushed.
What seems particularly unreasonable is to impose legal disabilities on the descendents of the host population in the name of a theory of systemic discrimination. It is one thing (and, I suspect, a necessary
thing) to create affirmative action rights for aboriginal people. It is another to create these rights for the descendents of voluntary immigrants.
Update: To be fair to Aristotle, he only says of the European barbarians, that "they are full of spirit, but wanting in intelligence and skill; and therefore they retain comparative freedom, but have no political organization, and are incapable of ruling over others." Pretty good, compared to what he says about the Asiatics. He certainly could never have made it as Appointments Commissioner.