Plummeting birthrates are only a problem for capitalists who want to keep the price of labor low. Even with the worst birthrate population crashes, there will still be enough workers to do all the essential work that needs to be done. It will require the wages for more important jobs to rise and for less important ones to fall. It will force businesses and governments to reorganize how the system works. However, it won't realistically lead to some apocalypse where seniors aren't cared for and everyone starves. We're getting that already thanks to disappearing socal safety nets, environmental destruction, rising wealth inequality, and political instability.
Productivity increases every year and yet most of us become poorer every year. All Malthusian arguments about population and fear mongering about scarcity are a distraction from the true culprit of our suffering: capitalism, imperialism, and the people in power bleeding the entire world dry for little marginal utility. We could technically be living in a utopic future, but instead we might see billions die to war, famine, and unmitigated disasters.
The worst part is that a lot of us do want kids, but either can't because of economic concerns, or decide not to because the future is so bleak. I'd never bring another person into this world if their future would only be slavery to the oligarchs.