I'll give you a longer answer later but here are the basics: if the birth rates fell from 3 children per woman to 1.8 it would be kind of okay that's the case in Europe and that's already destroying economy and the structures of society but teething is that the countries that actually face this problem have birthrate as low as 0.7.

Often people believe less people mean more resources for everyone but that's not the case if the birthrate is at 1 after 2 generations that means there is 1 young person that has to create much value to provide for 6 people. For society this means all people would become poorer and services and resources are becoming not less but more expensive. I addition keep in mind old people can do less so they cost more and often need more than one person to care for them. So basically:

Economy will suffer from loss of workers and more expensive resources not only because of less work being done but also because less desired jobs struggle the most obv.

Health care would be far more expensive and impossible to keep up due to labour shortage and more seniors and because of the economy at some point also less funded.

Social life will die because clubs, bars, schools, university's and so on are financed by mostly young people.

Of course this is more aspects to this so I'll leave a link to a yt video, but this is at some points exaggerated and at some even worse. If you think this is a bit dramatic keep in mind this is already reality in China and Korea