Thinking about it, does it? The Retail version of WoW is still going to be on the same numbers its always is, at launch and when a content draught comes, the people on the Vanilla server doesen't really do jackshit for the current playerbase.
What does blizzard get out of this? Nothing either. Much work for a measle maybe 200,000 people playing with an active subscription, then it's most probably going to collapse even more, and in the end people will get tired of no content at all and it would cost more keeping the servers active and creating it in the first place than what is profitable. Didn't the Nostralius team use
If we take into consideration that there is a legacy server for each expansion, there is no gurantee that people will play it forever, considering that there will never be any more content patches for any of them. Say it rises to 12 Million subscribers. How long is it before it all completely Collapses? Don't get me wrong, I'm not the ANTI legacy type, It love to go back to the glory days of Cata but WoW is moving forwards not backwards, and creating legacy servers could possible make the new retail even more dead