Yes, Western Civilization is in rapid decline and ultimately will suffer replacement from the inside out, outside in, or combination of both.
Should the Lord tarry, and this not be the final act, Christianity will continue however. Demographic population growth studies reveal that Africans will be the largest population group in the world between one and two centuries from now.
Here's what that looks like in perspective with Asia and Latin America:
Now Islam is growing faster than Christianity and Pew Research states "Islam is the only religion projected to increase faster than the world’s population as a whole." You can see what that looks like:
However, the point is that Christianity is rapidly growing in areas of the world experiencing the greatest population growth. And let's not forget that this happened before when Muslim populations expanded and Christian populations were decimated by the Black Death in Europe. Yet, Christianity rebounded and ultimately reigned supreme.
Either way; however, Christianity isn't going anywhere. Studies clearly show that should the Lord tarry, this latest iteration of Caucasian Euro-centric Western Civilization which cut its teeth in a New Holy Roman Empire, flowered during the Enlightenment, and reached it's zenith in the 20th century before falling into great moral corruption and suffering rapid decline will be a postscript in history in two centuries but Christianity isn't going anywhere.
"The whole of human history does not contain a single instance of a group becoming civilized unless it has been absolutely monogamous, nor is there any example of a group retaining its culture after it has adopted less rigorous customs." -J. D. Unwin (British ethnologist and social anthropologist at Oxford University and Cambridge University).