Well, like I said, if the trend is caused by something that's not easily reversible, and there's no negative feedback loop which would naturally cause birth rates to come back up in the future, then unless something happens to reverse the trend then mathematically speaking the end result of global birth rates below replacement rate is human extinction.

Granted that's not an imminent threat, it would take quite a few generations at current first-world birth rates. But I still find it a concerning long-term trend, and there are a lot of less severe negative consequences that could occur between now and then. If you care to dig into it more, this podcast episode has a good discussion of the short-term problems, which go beyond just elderly care: https://www.thepoliticalorphanage.com/p/the-great-baby-short...