The latter. Though it's not really even about a decrease or increase. Even if the population stays the same, the share of young to old will change drastically and with the way the modern economy is structured - it's gonna be ugly.
That is true, but it hits developing countries more.
Compare USA, UK, France (which are past the demographic transition and are having pretty decent demographic structure) with China, Korea, Japan (which are at various stages of that transition and are around or sub 1.0 growth (2.1 is replacement level) and almost no immigration).
China and Korea will have it much worse than any European country did (in EU the worst countries are around 1.3 and it's caused by in-EU migrations as well as demographic transition - so the missing .3 from Baltic countries or Poland is reproducing in UK, Ireland or Germany).
