In 1990s a small generation of Russians was born who are now in their 20s and 30s. This small generation (due to urban lifestyles and cultural reasons) will produce even smaller generation and Russian population will continue aging and declining
According to UN, Russia's population will be 135mln in 2050 but this scenario assumes increase in migration.
In zero migration scenario Russia's population will decline to 122mln by 2050, with ethnic Russians being 90mln people
So you do the math...By 2050 Russia expects 13mln migrants and these will be mostly people from Central Asia.
Muslims in Russia will be around 25 percent of the population by 2050.
Labor force---People aged 15-64---- are 96mln today, and by 2050 it will be 81mln, indicating decline of working age population by 0,5 mln people per year on average
Decline in workforce even despite migration means prolonged stagnation of economy and military weakness due to lack of conscripts and mobilizing potential.
Also, hardly Central Asia will be able to supply 13mln migrants because demographic potential of Central Asia is already exhausted and economies of CA republics are growing faster than stagnating economy of Russia-----so real population of Russia by 2050 will be much smaller and older compared to UN projections