Nature abhors a vacuum. Islam is seizing an opportunity in the West, as you say, Howard - although it's a slightly different, if related, topic to terrorism, of course.
Christianity is not dying out globally and is a boom religion in China, for example.
This is quite enlightening:
http://www.pewforum.org/2015/04/02/religious-projections-2010-2050/
As you can see, Islam is catching up. In certain areas that will no doubt happen faster than others.
