I don't think it's a bad example in this case, since the US hasn't lost it's own cultural heritage much. For better or for worse, the US does a great job of assimilating people and making them "American".

That's pretty much exactly what Quebec is trying to accomplish, right? something like 'if you want to live in Quebec, you have to become Quebecois'. So if US policy doesn't blanket ban other languages in signage and social services and still manages to 'americanize' people, then Quebec could potentially do the same.

The US and Quebec are in pretty different situations, so it's not a perfect example, but I think it is a pretty good basis for an argument against Quebec's culturally protectionist policies.