was fine with the 4 auction houses in XI.
I wasn't. It was bad then, it's bad now. Note that XI's seperate AHes have all been integrated now. All it does is inconvenience people. the City AHes only worked when the game was young and people still hung out in those places. Eventually, like with XIV, pretty much everyone

You can't deny that putting all the items in 1 pool (and effectively having 3x the amount of retainer space) would instantly erase the biggest frustration with the current system- not being able to sell stuff at all because you can't put your stuff up for sale at all except where nobody will be able to find it. The capacity for each city was designed with the thought that people would be evenly spread among the 3 cities- that didn't happen at all, and everyone piled into ul'dah because it was centrally located.

Even if we had XI's AH down to the last byte of code, having the AH in each city seperate is a bad idea. FULLY integrating the 3 so you can search all items from one location instead of haivng to check with 3 different NPCs that are each in different rooms would drastically cut down on the problems with both the current system and any future system put in place.