To which I instantly thought -- bet you none of those guilds had products of immediate military use.
Your best case scenario, in that case, is that the other guilds band together and start to demand that you churn out more fireball capable wizards. Why are you getting all the same treatment as the rest of the city's guilds, if you don't do what you can to defend it?
Worst case? A bunch of fireball capable wizards runs over your city. Not vitrifying it in the process, one hopes -- but your wizards won't be able to stop it. All it takes is one guild that decides that the utility of wizards is greater than their risk.
And what if no guild does? That bunch? It could be a red dragon. Or a horde of giants, or orcs, or wererats. It is, after all, a D&D universe.