Why Frostwolf Shouldn't Be Closed
I'll admit that the present and near future of Frostwolf look pretty grim. For example, my understanding is that the top pvp guild Devastation will be moving to another MoP server. I would expect to see Frostwolf reach very low numbers in the coming months and years.
I still don't think it should be closed, though, largely because the MoP server which a lot of Frostwolf people have been going to will itself die at some point when it moves to Legion. The server in question is a progressive server, and the devs there have already confirmed it will move to Legion at some point (they said they would skip WoD). It's also very possible that retail WoW will eventually offer the TBC and WOTLK expansions if Classic is a success.
When that happens, Warmane might well be glad that it kept its MoP server around. Maybe then they will decide to start improving and advertising the server in hopes of attracting new players.
Once you kill something it's dead- there's no reason to do that with Frostwolf. Just because there's very little interest in it now doesn't mean that will always be the case.