If it hasn't been updated for 8 years, then there's no reason to play them. Even a nerf or balance changes will cause more players to check it out. Else, dungeons are effectively dead
The problem is that, 8 years ago, dungeons and raids were "hard" and required "getting good". Now they aren't. We have Strike missions, raids, and other high-end PVE achievements to demonstrate how "good" a player can get. I would get into the game design aspects of it, but essentially not updating or streamlining the less popular content just confuses new players, and it's easy for them to get overwhelmed by the vast amount of content that exists in GW2. Dungeons, despite how old they are, have been untouched mostly, and take longer than they need to, for outdated rewards. Contrast this to Final Fantasy 14, where the game is extremely streamlined, and all players, new and old are properly incentivized to revisit old content via Daily rewards. It's just not worth the headache for any player to visit a dungeon, even if the rewards were boosted because it's just not worth the time.