It's a question of degree. I think the article is kinda hand-wringy, but not incorrect in the attention they're giving to orders of magnitude. You've got to remember that if a country like Spain halves in population, that's a colossal shift from A to B, to the extent that each decade will feel like a fairly major leap on its own. I don't think it'll be a death spiral, but it'll definitely absorb resources that otherwise could be directed to advancements, rather than preventing retrenchments. And that's assuming the rate of change doesn't cause nonlinear effects of its own.

Also, if Spain halves, it may well be that other countries shrink even further. You've also got to wonder about the distribution of old folk: some countries will end up with far more extreme distributions than others, but what I suspect we'll find is that, just as there's a quality of life level above which the reproduction rate decreases, there'll be a population level below which the quality of life decreases, so it'll self-correct.