You are right. As more arrive, the worse it will get.
20 years from now we will see a massacre a week on US soil.
if we even have 20 years.
If they continue coming at the rate they are now coming, I would cut that figure by 80% to "before the next Presidential Election" in 2020 (assuming we have one). If we can somehow stop the flow of Muslims tomorrow, I would agree with your current timeline. With the numbers now in this country, it is no longer a question of "if" but "when". More pouring in makes the problem bigger daily.
The history of the world for the past 16 centuries demonstrates that allowing Muslims into a country can only lead to destruction and death - or compliance by the host peoples with compliance with Muslim culture and laws (which also leads to destruction and death).
In other words, there is no conceivable sensible reason for allowing Muslims to migrate into a non-Muslim country except to ultimately change the culture and religion of that country into a Muslim one.