Here are the main issues of Transformice in no particular order that I think are contributing to its slow and painful death (and potential solutions) that have existed for years but apparently still are not being solved:

• Flawed moderation and rules: First things first: I will not state names, and also I have nothing against forum mods, or mods in general. However, mod corruption has been a common rumor and concern in the community. If Atelier makes the whole gist of being a mod more transparent, along with the selection process being a little more transparent, I feel like the community would trust the moderators more.
  But,
  I would say the current application system is too easy to lie to. Of course, any application Atelier comes up with will probably be easy to lie to. But I think I have an idea that could make things easier, rather than making people full-on moderators immediately, Atelier should have trial moderators who have more limited powers (maybe muting and such but not being able to ban). Have current full-mods judge their performance and decide whether or not they should become an actual mod. I feel this would help get rid of some 'corruption' within the moderator team. I also am not a mod and have never been a mod, but perhaps mod training and protocols for certain rulebreaks or situations would help a bit more, unless these are already things. These are only suggestions and things that I have seen other games do, so I suggest Atelier to take my suggestions on this matter with a grain of salt.
  
  Additionally, one of the rules in particular seems redundant: "Insults to the staff" - I feel this rule is not necessary, as "Severe Insulting" is already against the rules. While I understand that moderators are human beings who have agreed to volunteer large amounts of their time to moderate a game with thousands of players on a daily basis, I feel like they shouldn't be placed on a higher pedestal like this because then they can use their powers unjustly. If I can be insulted lightly, then a moderator should be able to as well. I shouldn't be 'rightfully' subject to a mute or ban for jokingly calling a moderator a noob (although I feel most mods would not mute or ban me for that, they probably could with this rule in place), when I can jokingly(or even seriously) call any other player a 'noob' and moderators will not bat an eye (because it's not against the rules). This might be a bit of an extreme example, but it shows the extent of this specific rule.
  
  
• Repetitive Gameplay + Lack of New Content: Transformice has had the same official gamemodes for at least 6 years, and new events that aren't being reused are becoming rarer. The design of the game is rather limiting which can explain the lack of gamemodes, but can't Atelier think of something? I'm not going to make any suggestions here because this is not the place, but Atelier's players have plenty of ideas and surely they have, too? Maybe they could use ideas from the many other games they've made, if they can't think of anything?
  
  Listen, people just want more content in Transformice. It doesn't have to be super big, but something beyond new accessories and furs, something that actually affects the game, will please the community at least for a bit. I believe that 'new content' is what people want most in this game. It has not changed very much in the 6 years I've had this account, except for maybe visually, and that is such a shame.
  
  
• Lack of Advertisement: This is pretty self explanatory. Atelier's games have failed or are failing like Transformice because they refuse to advertise them in any manner that is not word of mouth from their current players. Advertising the game will help keep it afloat for at least a while longer thanks to an influx of new players. To keep old players, maybe refer to the points above.

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If it is the fact that Atelier no longer has the passion to develop Transformice, maybe they should sell the game to another company. It's pretty clear to me that they made a simple game that blew up unexpectedly, but then realized that the game they have made is not the game they want to primarily focus on and develop for many years, but they are forced to do so because it is the game that is the most popular and generates the most revenue. Maybe I'm wrong, but if I AM wrong, maybe they should think about why players like me feel that this is the case for Transformice.

Tldr;
Try to improve the moderation system a little bit (at the very least be more transparent about what being a moderator is like and how moderators are chosen), genuinely update the game, and advertise the game, or sell it while you still can, because as far as we know, you guys don't care.