A gradual decline in the birthrate to slightly below replacement level is not necessarily a bad thing. A sudden and precipitous drop to far below this point, as has happened in most European countries, is disastrous.
There are a bunch of problems. The most obvious will pop up 30 years from now, when a large number of retirees will be (not) supported by a much smaller and shrinking contingent of workers.
Combine this, in the case of Europe, with a large and growing group of unassimilated immigrants, and you are facing some major problems.
European countries, if viewed as organisms, are in the process of committing slow-motion suicide.
The US is in somewhat the same position, though to a lesser extent. Our white middle classes have a higher birth rate than Europeans, but what I think will save us is that we do a more effective job of assimilating immigrants. (Although our record here is not what it could or should be.)
Why should the workers of 30 years in the future support the generation of today? Aren't people saving for the future? Are they all buying into the lie that social security is some sort of savings program? Are you saying that the socialistic concept of "from each according to his ability ; to each according to his need" be applied to those who currently squander their wealth and will hence be "needy" in the future? DO you think that just because someone pays welfare taxes now (FICA) that they are entitled to receive welfare in the future? I hope that the productive workers of the future line Kongress up against the wall and institute Romanian term limits if they shift any more wealth away from the ones who earn it to those who whine for it.
I grew up when the population of the country was in the 170 million range. Technologically we're much more advanced today - I like having computers, advanced medicine etc., but the vast loss of freedom as government is 1000 times more intrusive today than then and and the vast loss of wealth as bureaucrats plunder far more of the wealth of the nation to pay for their worthless social engineering schemes make the quality of life not as good in many aspects as it was then. This cancerous government growth is fueled in part by the rising population which has lost a sense of personal responsibility and in turn expects their wants to be meet by the labor of others. Look at where socialism is at it's worst. It's in the most densely populated states and areas.