I just don't get why Blizzard won't give it a try. It works well for Everquest with their progression servers. For those who don't know, EQ progression servers start as it released in 1999 and progress through content like the game did at the start. The rate is a little quicker than it was on real release, but still a decent amount of time per patch/expansion. They make a new one every so often so people can replay it or pick it up or whatever. Don't like WoW until WotLK? Wait until it hits then pick it up. Don't want to deal with Cata? Quit until the new server comes out. Once the server catches up to live content, it can easily just go away. Offer a free xfer or something for all progression server chars so they can join their main server, or go to a new place with their progression server guild etc, then close the server down and restart it.
They could have 2-3 progression servers running at all times. Not a huge money strain on server hardware, would let people join up at random points of the game that fit their liking etc etc.
"We don't think people would stick with it and play for a long time" is Blizzard's most common response. My counter is people aren't staying and playing for a long time with the live game, does that make it not worth having?
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They could easily just have set rules for it. All content lasts 1/3 the length that it did on live. So that way people know how long until a patch, how long until an xpac, or whatever they want. It wouldn't be dictated by on server play or anything.