The servers aren't dying because people are reaching endgame, that's a biased statement if you consider getting good armor/elytras endgame. Towny & Factions (if setup properly) adds extra elements to the game such as managing a town or base raiding. Unfortunately, every faction revision has ruined the server by having insane world borders which makes finding other factions an impossible task. Towny on the other hand has consistently retained population from those who enjoy building and the vanilla survival experience minecraft has to offer, but it eventually dies off because people get bored.
The problem with minecraft aside from botched setups is that xG lacks players who enjoy raiding or building enough to sustain the population of our servers. We're also not gaining any new players, there are too many established servers that offer a better experience than our out of the box factions & towny.
Mod packs is probably the right move to keep interest for longer but requires more resources and setup than your traditional spigot server setup. You also need a good computer to handle some modpacks so that might restrict a few interested players. Regardless I think its worth trying to setup, obviously factions/towny hasn't been working out so maybe its time to try something new........