> What does it mean to say "we were promised flying cars"...

I don't know about the other things you mentioned, but I think you have this in the wrong category. "We were promised flying cars" is one half of a construction contrasting utopian promises/hype with dystopian (or at lest underwhelming) outcomes. I think the most common version is:

> They promised us flying cars, instead we got 140 characters.

Translation: tech promised awesome things that would make our life better, but instead we actually got was stuff like the toxicity of social media.

IMHO, this insight is one of the reasons there's so much negativity around AI. People have been around the block enough to have good reason to question tech hype, and they're expecting the next thing to turn out as badly as social media did.