Sandox itself is very good mode, its actually one from the best modes in garysmod in my opinion, just under DarkRP and other RP modes. Sandbox is not boring or bad but people playing on the servers and administration. If the server is left alone with no regular updates its obvious that it will get boring sooner than servers with regular updates. Anyway, everything gets boring in some time, no matter how good or bad the mode or server is. We must say that Garrysmod is a dead game and its obvious that the modes and servers will be boring.
I don't see a reason to play sandbox. If I want to build something cool I go into single player or go on to my server. If I want to kill people I do raids on rp servers. And if neither of that is enough, I'd just go to another game. I don't want to wait 45 minutes to download 20gb of garbage mods to then listen to screaming kids.