The report says that in 2050, Muslims will make up 10% of the European population. But they will number only 2.1% in the United States.
Interestingly, the study also says that the number of atheists and non-religious affiliated will decline globally.
This may very well be, but, like many statistical studies, it is mainly just an extrapolation of current numbers over time. Muslims have a higher birth rate than Christians do, so if we graph that out, their numbers will be higher by 2070.
Other scenarios are not factored in. For example, what if some of the 10% of the European population that has an Islamic heritage convert to Christianity, now that they can be exposed to it? That may depend on Christianity reviving in Europe, but that is not outside the possibility of the grace of God. Or what if the brutality of ISIS and the Islamic terrorism that is rampant in the Middle East creates a reaction against the religion? Or what if the Westernization of Islamic countries creates a decline in the birth rate? Or what if the Christian birth rate shoots up?
Lots of things can happen, there being many more variables and unpredictabilities in life than a single statistical trend.