The term automat refers to a model of food distribution. [0] The movie [1] is about a specific firm, Horn & Hardart [2]. I don't think I'd agree with a thesis that suburbs were the direct cause of H&H's automat chain's decline. Logistically, there is a balance of preparation, freshness and transaction latency. The automat inverted freshness and transaction to prep latency from trad restaurants. The arrival of "fast food" used standardization to decrease prep and transaction latency from trad restaurant levels while maintaining freshness levels. In terms of payment tech, I'm not able to find a reference but my recollection is that automated bill accepting technology not common until the 1980s. If automats were unable to accept bills then they would be unable to offer higher value meals without human intervention (and lose their transaction latency advantage). [3]
The format was threatened by the arrival of fast food, served over the counter and with more payment flexibility than traditional automats. . . . Another contributing factor to their demise was the inflation of the 1970s, increasing food prices which made the use of coins increasingly inconvenient in a time before bill acceptors commonly appeared on vending equipment.
0. https://en.wikipedia.org/wiki/Automat
1. https://en.wikipedia.org/wiki/The_Automat
2. https://en.wikipedia.org/wiki/Horn_%26_Hardart
3. https://en.wikipedia.org/wiki/Currency_detector
Edit: removed first sentence "It's like a failure of imagination is cyclical." which had to do with H&H and other's ability to pivot, which H&H did in spades.