You imply that the technological development will stop, but that's not what happened to flying cars - they do and could exist. Since the 1930s the technology didn't stop developing - aircraft went jet powered, supersonic, to the edge of space, huge, light, heavy, more efficient, more affordable, safer; cars went faster, more reliable, safer, more efficient, bigger, smaller, more capable, self driving; fuel got more pure, engines got more power per kilogram, computer aided design and modelling of airflow and stress patterns was developed, stronger lighter materials were developed; flying cars have been built:
Klein Vision AirCar, 2022: https://www.youtube.com/watch?v=5hbFl3nhAD0
Predecesor, the AeroMobil from 2014, from prototypes starting 1990: https://en.wikipedia.org/wiki/AeroMobil_s.r.o._AeroMobil
The Terrafugia Transition 'roadable aircraft' from 2009: https://en.wikipedia.org/wiki/Terrafugia_Transition
Moller SkyCar, of course, which never got to free flight, but was built and could fly.
The problems are regulatory, cost, safety, massive amounts of human skill needed, the infrastructure needed, the lack of demand, etc. A Cessna 182 weighs <900 Kg, a Toyota Corolla weighs >1400 Kg and has no wings, no tail. But if we collectively wanted VTOL flying cars enough to put a SpaceX level of talent and money at them, and were willing to pay more, work harder, for less comfort and more maintenance, we could have them. Bit like Hyperloop; a low-pressure tunnel with carriages rushing through it is not impossible, but it's got more failure modes, more cost, more design difficulties and almost no benefits over high speed train and maglev.