It's no secret to anyone that the playerbase on PZ is dwindling. Let's be realistic, it was only a matter of time until another server made a good version of Gmod's Murder, and it's happened now, on a few large servers. So, how do you cope with that?
Well, first off the server needs competition. Right now it's really easy to get tired of the server since there's no real long term goals, which I'll talk more about in a bit. One fantastic way to foster competition is monthly/weekly/daily leaderboards. Put up a (much more public) area where people can see all time top murders, monthly top murderers, weekly and daily top murderers, and you've already given hundreds of players a massive incentive to spend a ton of time in game. Do the same for downgrade and maybe even party games, and whoosh, constantly more people online. The best part? It doesn't even have to be an event, or even have rewards beyond bragging rights of who is top on the board.
Secondly, you need a way to bring in new players. An easy way to do it is after introducing the leaderboards I suggested, you invite a few youtubers with an *older* audience than the server is used to. The current server average age is probably somewhere around 12, and that's no good: 12 year olds rage when they lose, rather than thinking "how can I get better?" As someone who's spent a lot of time getting good at PvP, you'd be shocked at the amount of hack accusations I get in downgrade simply because I know how to aim and strafe. If you up the age to maybe 14, 15, you get more mature (ish) people who are on average better at the game.
Third, as I said before, the long term progression on the server isn't very good. The main/only thing to do that gives a sense of achievement is ranks in murder, and those are slow (many games to get to diamond for a new/average player, hundreds of hours for a top player to even get top100). For that, I suggest a few things. One, a better achievement system. You could reward players when they hit certain milestones: 100 kills as murderer, trade for 10, 50, 100, 1000, 5000 guns, etc. Even if the reward is only coins, you're providing that "I did it!" sense of dopamine, which is psychologically much more likely to make people want to come back for more.
Fourth, and this one will probably get a lot of backlash, you need to tone down the rules. The warning system is horrendous: how many people have actually stuck around to get the 30-40 warnings that equal a ban? Only really dedicated players, and that's exactly who you don't want to be getting rid of. So, filter words better (People much prefer an automated system that stops them from cursing) and have staff be much more lax about punishing people for being a little rowdy. If you want a family friendly server, you're not going to have a very successful server- bigger servers are less moderated, and this server is currently far, far overmoderated.
This will help keep around those older people who don't want to feel like they're playing a server designed for 10 year olds.
Lastly, discourage basically everything that currently goes on in creative chat. If the steps above are followed, it's unlikely that most of the competitive players will visit creative. However, if they do, the constant "L()()K RP HERE JOIN MY RP MYOC RP HIGH SCHOOL RP PLS JOIN" will make people vanish faster than you can say "wow you're roleplaying on minecraft? seriously?". Make a separate chat for RP advertising that's only accessible via some command that enables rp-finding-chat, like a separate thread, and disable it by default. Will some roleplays suffer? Yeah. Will creative chat benefit hugely? y e s s o m u c h o m g. Similarly, in creative filter out typical words that people use to advertise roleplays unless they're in the roleplay advertising chat, so that people don't misuse the main chat.
The server has catered to young children for a really, really long time. Now those kids are growing up, and seeing that there's nothing for them to do on the server: Murder? Takes ten games to change a single percent on their rank. Downgrade? Nobody ever plays. Creative? Not everyone builds, and chat is disgusting there. Elite survival? Huge paywall + still nearly nobody ever plays.
If you don't want the server to die, please make it more competitive. It's how servers are successful, for the most part. Look at Hypixel and Mineplex. There's achievements over time for every single game, unlockable skins and characters to play in most games, which is far better than percentage increases in invisible perks like in murder, there's monthly leaderboards so people want to compete, and they're absolutely monstrously large servers.
//rant over