Well I started at release on Stormrage Horde. It was a dying server by the end of BC and dead as dickens by mid-cataclysm. That said it is the #1 Alliance server by population and if somehow you shoved a pile of other dead horde servers together to it you'd just bring down the server. The other issue is the only people left on dying servers are NOT the proper population to "bring it back to life". Ultimately those people left on Stormrage Horde (or any other completely dead faction/server) are not going to magically organize into a series of stellar raiding guilds that can attract successful players from other realms ... that's just not who they are. Anyone with that kind of ambition found a way long ago to go anywhere else (my entire guild moved off SR-Horde, it sucked but we wanted to survive).
And therein lies the real problem. The answer is very complicated. If you wanted to revive SR horde (or any server like it) you'd have to merge their population with an already successful population from another server.
The other issue, however, is that if you make transfers easy you simply get too many people. Stormrage Alliance is actually not fun to be on. Yeah the population is great but stormwind is overcrowded like Shanghai ... it's NOT a good thing. Honestly they need a two-pronged approach here. They need to take dead servers and merge them into already alive ones. Then they need to find a way to cap/limit how many people can exist on a one faction. Honestly you could split Stormrage Alliance into two servers and both would probably still be in the top 20 for population at this point. It should have never been allowed to get there.
One server is also no solution as you completely erase any possibility for player-society beyond your guild. It works for smaller games but in a game this large you need to restrict the active populations to a consistent population size.
Ultimately the answer is basically "somewhere in the middle". You want a mid-large population on both factions of a server and you want to employ mechanisms that keep it that way. Their organic/laissez-faire policy has definitely NOT been the correct answer and from all the blue posts I've seen about it over the years I believe they concur at this point.