As much as I think the developers hearts are in the right place, I don't know if there's any easy answer to a very good question. Your question leads me to the additional question of "What can be done to increase the number of players?", and I don't have any answer for that at all. The numbers playing seem fairly stable, which is very good. Growing the head count is tricky though, for a couple of reason.

-Most of the new players are returnees rather than completely new players, and that's a relatively small market to tap into.
-It's difficult for Utopia to sign up brand new players given the lack of flashy graphics and very steep learning curve.
-It's not really a game designed around mobile gaming due to its text and math-based nature. A dedicated app may help, but that's likely a huge amount of work.
-It's very tricky to balance the game between new players who won't know what they're doing and old hands who do. Having the latter target the former (which, dog eat dog, will typically happen) without having the former just give up is a difficult tightrope to walk. Teaching a new player about the game takes about half an age, even if they're attentive and willing to learn, and the teacher can get burned out by that.

However, what's reassuring is that it's a longterm goal of the developers and the vast majority of the players to increase the player base. It's also important to note that with the integration of Munkbot with Slack, and the huge strides have been taken to improve the experience of playing the game on mobile devices, there is work being done. Communication apps like WhatsApp and Slack also help with keeping in touch with ex-players easier, which makes it easier to harangue them into coming back.

I think Utopia's greatest strength is its core tenet- It's a team game. It's what resonated with the thousands of people who played before and it's a core reason why many of those that still play do so. It has it's own (occasionally unpleasant, often fantastic) subculture and you get out of it what you and your kingdommates put in. That's what I'd market if I were building an advertising campaign for the game. I'd also probably emphasise the fantasy nature of the game. A wiki is nice, but something more "magical" would probably encourage RPG lovers to give it a try. People often come for the lore and stay for the gameplay.

But like I say, I don't see an easy way to grow the player base. I do see steps being taken to try, and I wonder what the players could do to help. The developers make the game, but players make the game special. We have our part to play too.

I'd love to see a "general release" Genesis server as well.