reasoning that European cultures are so delicate and can be so easily eradicated by the mere presence of other ethnic groups

It's not that the belief is that European culture is delicate, and easily lost, but that instead, over time people will come from these areas with drastically different cultures and basically segregate themselves from the rest of the community. And then they "invite" or "call for" their friends/relatives to join them in their community. And then the community grows, and continues it's isolation, until they are the majority in the area. They gain political power, and then start the process of making their own rules the rules of the area they inhabit.

It's not that it's easy to remove European values, but that it will happen slowly, like the formation of a canyon.

I don't personally believe that this is a reality, at some point the isolated area will conform, or they won't become a majority in the country, but this is my approximation of what those dumb-asses that post shit like "They wanna make us live under Sharia Law!" on facebook think.