If there wasn't a fertility decline in the Western world, there wouldn't be a lopsided ratio of retirees to new incoming workers.

That future probably wasn't terribly obvious to the politicians who created them (given that most public pension plans were created roughly between World War I and a bit after World War II).

In fact, based on some of the literature of the time (eg The Population Bomb -- https://en.wikipedia.org/wiki/The_Population_Bomb -- etc.) I'd say it's more like people were fearing the reverse.