Let’s say that 50% of the Classic playerbase will want to transition to TBC. 50% of every guild and every server will copy their characters and play TBC. Now, imagine what that does to a Classic server. Half of your raid team gone, half of your community gone, servers that had healthy populations now underpopulated. In response, some guilds will likely make executive decisions to move to TBC in order to stay together, taking Classic players with them for the sake of the guild.
TBC will gut Classic if we just have character copy. What we should do is have:
Classic servers that stay Classic.
Classic servers that transition to TBC and then Wrath.
Most of the 50% who want Classic only will roll on Classic only servers. Most of the 50% who want to progress through expansions will roll on progressive servers. When TBC launches, Classic-only servers aren’t depopulated.