Not all of Ul'dah is corrupt to be clear. The humble merchants and civilians of Ul'dah aren't (all) corrupt. It's the Monetarist who are and they are people leading Ul'dah. The Sultansworn and Immortal Flames aren't corrupt but the Monetarist have near infinite means into manipulating, misdirecting, and straight-up misleading them into doing what they want.

The Ul'dah we know no matter how corrupt is important to Eorzea's sovereignty. Ul'dah was at Cartenaeu and helped during Operation Archon. They are a very important trade partner for Limsa Lominsa and Gridania. Ul'dah is apart of the Eorzean Alliance. Destroying it would have serious consequences for everyone without including the fact the Garleans would undoubtedly try to take over (and most likely succeed). The corruption within Ul'dah needs to be dealt with but you just can't just beat it out of it you need to be careful or else you risk making things even worse.