Why would it be abandoned? Even in the world of magic, transportation was fairly important in G4. Now we have a world without magic, and they decided that they would drop the non-magical transportation? Why? This makes even less sense in Zephyr Heights where you have verticality, and you can't fly anymore. Your city isn't self-sufficient. You will need to access the ground sometime, and you sure don't want to walk it every single time. At the very least, the royalty would have ballon travel to reinforce their importance while they were trying to pretend to still have magic.
The only thing I can picture as a reason is that the fire mechanism that provides heat may be magical. There is no evidence that I can find in the show for that, but maybe.
At which point, the ponies that supposedly invented the smart phone should probably be investing something to fix that problem. Not only are their trains, which must work through combustion or electric engines, but there the movie had a tank. Are you telling me that you are in a world where you can't figure out how to make a controllable open flame to fix your balloons, but you do have cellular connection readily available across the world?
Unless that isn't cellular… don't tell me you have created satellites for global connectivity and the capabilities to launch them, but you can't make a hot air balloon.
I'm not giving you any help.
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It's even stupider than that. There were hot air balloons in the movie.

your current filter.The only reason they could have abandoned them is they just went out of style.
The only thing I can picture as a reason is that the fire mechanism that provides heat may be magical. There is no evidence that I can find in the show for that, but maybe.

your current filter.At which point, the ponies that supposedly invented the smart phone should probably be investing something to fix that problem. Not only are their trains, which must work through combustion or electric engines, but there the movie had a tank. Are you telling me that you are in a world where you can't figure out how to make a controllable open flame to fix your balloons, but you do have cellular connection readily available across the world?

your current filter.Unless that isn't cellular… don't tell me you have created satellites for global connectivity and the capabilities to launch them, but you can't make a hot air balloon.