But the fear isn't so rational. It's like a fear that the cocktail in your example will replace the original vodka whether they want the cocktail or not, or that the vodka will be so diluted by seltzer that it will functionally cease to exist.

It's like a fear of gentrification of the country as a whole.

It's also important to remember that the US is a huge exception in this regard as well. Most other countries are like 90%+ native population, and immigrant populations tend to be sort of isolated from the wider national culture due to things like language barriers, and they often set up little "bastions" of their native culture locally wherever they live. We even see plenty of that in the US as well. While there are many distinctly US cultures across the country that are derived from a variety of backgrounds, there are tons of "enclaves" of European culture that make it blatantly clear where immigrants from certain countries settled. In Boston, the culture of Chinatown is distinctly unique and separate from the wider culture of the city, which largely has ties back to Ireland (and is very proud of it). And both of those are distinctly different from where the Italian immigrants settled, who effectively have their own districts of cultures descended from Italy regardless of where they immigrated to.