I think they were serious, and very much a product of the malthusian ideas that dominated that era. I remember reading futurist books with these images in the

I think they were serious, and very much a product of the malthusian ideas that dominated that era. I remember reading futurist books with these images in the 6th grade in the early 90s. Back in the 80s and 90s Malthusianism was the dominant thing. Most people believed the global population would keep expanding beyond what the world could sustain giving the impetus for things like space stations and ocean colonies. And population graphs in the 70s and 80s and even the 90s suggested that this was true. Also in the 70s the idea of the green revolution wasn't fully cemented in the public consciousness. Nobody had any idea that it would double wheat yields between 1970 and today so people were projecting slow linear growth, or worse static farm yields and available resources against a growing population. The projection for the future was a dark one back then, and things like these space stations, along with things like ocean colonies were seen as a desperate way to prevent that fate.