Thanks for your suggestion!
The note of a "temporary fix" is indicative of why we haven't done it. As we've shared, we don't believe in designing (and redesigning) features to accommodate shifts in population dynamics, especially when considering them temporary. Regardless, however, we design features for the long-term and in the image of the server we want, not the server we have at any particular moment, because the server, its population, and the interests of that population change over time, which makes it incredibly impractical to be constantly revisiting features such as this.
This request comes up periodically when population falls or certain guilds/cities see changes in their demand and attempt to preserve their city. Each time we say the same thing: We have evaluated the thresholds for having a shuttleport and other city perks, and feel comfortable with their current levels regardless of population changes and therefore don't intend to make any adjustments. Each account can have three characters; for accounts with Jedi, that is soon to be four characters in total. The number of people you need to maintain a city with a shuttleport is already small. If your city is struggling to maintain its population, I would suggest instead focusing on recruitment and membership, both for the server at large and your city if you need to maintain a population threshold you aren't currently sustaining.