The server isn't "new" anymore. Of course it's not going to be as popular. Warmane could have the same level of staff and dedication as Blizzard and make everything work perfectly, and that'd help some, but not really. Mists of Pandaria is old, and doesn't carry the hype Wrath of the Lich King has, so no matter what happens, it'll still be MoP, people will still join in small amounts, get bored and move on, and Frostwolf will still feel empty.
The real question is, how will you deal with it? Would you still continue to play on the server even if it's not popular and bustling with people everywhere? --because that's how it's going to end up no matter what Warmane does.