I think you're smearing missionaries a bit - they haven't ALL engaged in forcible conversion of natives. Quite often it is a voluntary decision.
But anyway, my point is that culture changes, and you can't stop it. Think of it as a culture free market. You can try to put up protectionist barriers to stop it, but you won't succeed. Not even the Amish are totally successful at it.
I grew up in Redmond, WA in the 70s and I don't recognize the place anymore when I visit. It makes me a bit sad, but that's life. Someday miles-high glaciers will form and scrub the Rocky Mountains away, too.