I don't think the server would be crashing. During Stormblood there were usually more people in Rhalgr's Reach then there were in Ul'dah or Gridania, showing off their latest Doggo, Savage-Mount or whatever. Also they could always make instances. If there would be too many people with mounts in a city: just split them.

That being said I don't really want mounts in most of the cities, but I could actually see them in Gridania. Every time I'm visiting Jonathas I just wish I could ride a mount there.