To be honest, Subway is probably the worst of all fast foods.
Here in Russia it sits idle at any food court housing any other fast food franchise.
It is only popular it places lacking any other budget food chains at all.
On the other hand I fully understand people jumping into their franchise because it is much easier technologically.
It is extraordinary easier to run as far as you don’t need to fry burgers and so on.
I avoid Subway unless there is absolutely no alternative. Huntsville, Alabama airport generally only has (or had) one “restaurant” open, Subway. Yuckkk! I learned to eat before going to the airport. Been stuck for time once or twice and wound up in a Subway. Awful food. I could definitely loose weight on a Subway diet.