Viva pretty much hit the nail on the head with his points he made in my opinion. Although I wouldn't really say GMOD is a dying game, or at least that really isn't a reason on why MRP/SWRP is lacking in numbers.
MRP is essentially a TDM server with light sprinkles of "Roleplay" in the mix of it. There's only so much you can do before it gets stale and people leave, like Viva said most people only stay on for wars and then go play something different when peace time occurs. The only things people can do during war is either host/participate in tryouts, sims, or some bullshit training which literally no "regular" recruit even likes doing. Of course there is the rare event some people plan out, but even those get boring/repetitive because there's only so much a regular player can do.
I personally think the way PVP works/wars in general needs to be reworked/revamped. Having only 30 minutes of TDM war and then having over an hour of peace time gets so boring the only reason people really stay on the server during peacetime is for tryouts (which in reality are stupid boring themselves, why the fuck do SOC officers feel the need to make them last for 1-2 fucking hours?) or to talk to random people in base who are equally as bored.
There's also the fact that there literally isn't a point in becoming an officer anymore when there isn't any real "reward" to it. Again, I know I bitch and moan a lot about reserves being removed but there is literally no point for officers to stick around or even try to decently run a faction when they know they aren't going to get jack shit from it. You need to give people some sort of incentive to stick around and actually care for the faction they lead, whether that be reserves or some other way of rewarding them. Not sure if you saw this thread or not but if you haven't I recommend reading it as it suggests a lot of balances and whatnot to reserves.
EDIT: Just saw your latest post, not sure if any sort of system like that was implemented yet for reserves. Definitely should be something Jim or whoever is a manager now should think about if it hasn't been implemented yet. Most of the community wanted it to return in the first place.