I got my hands on a demographic forecast for New York State broken down by county from 1968 that goes all the way out to 2020.
They really were optimistic. They forecast 26 million New Yorkers by 2000 and 31 million by 2020. In reality, it was just 19 million in 2020. The biggest misses were in the New York City Suburbs. They forecast 2 million for Westchester County (reality: 970,000), 287,000 for Putnam (98,000), 814,000 for Rockland (325,000), 1.1 million for Orange (385,000), 2 million for Nassau (1.3 million), and a whopping 4.7 million for Suffolk (1.5 million). Population forecasts for Fairfield County and Northern New Jersey were almost certainly even more overly optimistic.
This gives you a look into the thought process for when they drew all those lines.
Predicting exponential growth is hard! Plus their methodology was to basically take the 1950s and assume it would happen six more times in a row, which is a problem. This is why I think any "This is what society/place X will look like in 2050" etc. articles are crap.
A demographer in 1968 would see that, accounting for child mortality, American women had pretty much always had 3 children on average and conclude that the low fertility period between 1924 and 1940 was just a one-off. Also, they didn't foresee the near total halt in black migration northward after 1970 due to the civil rights laws, which had been a major source of growth for New York.

Also, the massive number of people moving to Florida really caught them off guard. If New York State had only grown as fast as the country from 1968, then by 2020, they would've had just under 30 million people, pretty close to the forecast. As it was though, they only grew by 8% to 19.5 million thanks to outmigration.
Predicting the far future is always a challenge. But it's ultimately necessary. It's better to have too many highways and reservoirs than too few.