Muslim birth rates are declining also, but demographics is a game of last man standing. And right now the last man won't be Western.
BIRTH RATES OF MAJOR, PREDOMINANTLY MUSLIM NATIONS:
Algeria 7.37 (1977) -> 1.86
Bangladesh 3.85 (1991)-> 3.09
Egypt 6.05 (1977) -> 2.77
Indonesia 5.76 (1966), 4.68 (1979) -> 2.37
Iran 6.2 (1986) -> 1.72
Libya 7.96 (1973) -> 3.21
Morocco 5.90 (1980) -> 2.62
Pakistan 6.92 (1987) -> 3.86
Saudi Arabia 7.59 (1974) -> 3.94
Syria 7.77 (1981) -> 3.31
Tunisia 4.51 (1984) -> 1.73
Turkey 4.87 (1978) -> 1.89
A lot of people presume France's birth rate is rebounding because of the Muslims in France. This is not true. Notice that French colonies such as Algeria and Tunisia have fertility rates LOWER than France.
These nations will have (or, have had) their birth rate drop below replacement levels (2.10) in the following years:
Algeria, 2003
Bangladesh, unknown
Egypt, unknown (c. 2040?)
Indonesia, 2017
Iran, 1998
Libya, unknown (c. 2040?)
Morocco, 2020
Pakistan, unknown (c. 2040?)
Saudi Arabia, unknown (c. 2050?)
Syria, 2030
Tunisia, 1998
Turkey, 2001
(The following are nations with large Muslim populations that were excluded because they were not predominantly Muslim: China, India, Nigeria, Sudan. Iraq and Afghanistan were excluded because of ongoing warfare.)