You are right to some extent. The political repercussions are only beginning. If they remain illegal and real attempts are made to stop the flow and reverse it the political changes are not drastic. An amnesty or perceived amnesty will ratchet up the flood tremendously and will eliminate the Republican Party from contention in national politics and prevent any sort of Conservative party from taking its place. Illegals vote already in all Democrat controlled political districts and they vote Democrat. Amnesty will increase the flow of new illegals and raise the percentage who vote. Actually the US is still above the population replacement minimum and one large segment, orthodox Christians and Protestants are actually increasing birthrates.
Not based on what I've seen which puts the US at 2.09 where the neutral (population neither increases or decreases) replacement fertility rate is 2.1. Given that the "boomer" rate was 3.8 no way is 2.09 going to pay for the entitlements for the very much larger boomer generation.
I can't locate anything on birth rates broken down by religious groups but I do know that, at least in Europe, the Pope was issued alarms about the decline in birth rates among Catholics.