Some people in power do dissent, as I said, the CSU leader, Seehofer, has objected. And this has had some effect, remember that Merkel has now accepted that there can't be unlimited immigration and there's a need for limits - up to 500,000 a year - so it's not free-for-all that's being intimated.
You say that the low birth rate is a real issue (which it is: there won't be the workers to support the welfare and pension structure in about 30 to 40 years time) but how is it going to be fixed? There are only two options: more births (and you can't force people to have kids) or immigration (there is a third option: dismantle the welfare structure but German pensioners are well looked after and there'd be howls of protest on that). The low birth rate is something affecting many European countries (as well as Japan and China) and politicians have rather belatedly woken up to the fact that this is an issue
500,000 a year! That is still absurdly high, Gwylan!
As for the low birth rate, that is exactly the sort of thing you want the government of the day to be tackling with sensible economic policy. I'm sure there could be tax incentives that would do the job. Or perhaps innovative change in child care - nearly all kids in Germany go to kindergarten but they don't go to full time school until they're six. Certainly there is a real danger of creating more problems than you solve by allowing an influx of immigrants. Let's look at the numbers - 500,000 people. How can a country realistically check they are all safe? What about when they get here? Are they all going to go to the ghost towns of East Germany when they arrive or are they going to go to where the jobs are?