While it's true some ghosts would leave, I expect that they would accumulate at a higher rate than they deplete. After all, as they don't die, they would mostly be motivated by unfinished business and avoiding boredom. Having lived mostly in Ishka, thats where the majority would have any unfinished business. Also Ishka would likely be among the most interesting places (or at least one of the easiest places to consistently find interesting things). Then there are the ghosts that decide to take up jobs as ghosts.

Also there are places like Ether, the district scattered throughout the city but in the ethereal plan and populated solely by ghosts.
Even if they rarely are seen, I expect that the number of ghosts is quite high and could easily account for around 30% of the population. Especially given how old Ishka is.

Also, thinking about it, an idea for an organization (better name would be welcome).
The City Wide Incorporeal Gambling Syndicate
Given the large number of ghosts in Ishka, and how long they persist, many come to find that all they have little to do, and thus strive to find interesting things to keep them occupied with their death. For that purpose the IGS was formed. Consisting solely of ghosts, most of its members no longer have any particular steak in the happenings of the living. However, to avoid boredom they make bets and wagers on interesting events that are going on. They also keep each other informed and observe things throughout the city. Usually they avoid interfering, but that is more out of curiosity of how events will happen on their own rather than actual rules.
While they lack any actual things to bet, they instead have a point system that they use. A members rank is determined by how many points they have, an many ghosts have since come to make note of the points even when they don't take part themselves (It has even caused living to make note of it due to how much weight it can carry among the ghosts of the city). A ghost upon joining the IGS gets 1000 points to use. Often they will have multiple wagers running at once. If a ghost runs out of points due to bad wagers, they can either attempt to borrow them from other ghosts on those ghosts terms, or take a loss point and get another 1000 points. A loss point is generally considered bad to have causing most ghosts to try to avoid that choice, but many have one or two from when they first started.
There are a few ghosts who are in charge of keeping the records for the points each ghost has. All wagers are reported to them.

Owrtho