Not really...you think if in a few years down the road there are only a few thousand people playing vanilla that Blizzard is going to pay to keep all the servers running if they can't make a profit off it? No.
The only reason they're doing vanilla is because they will make a profit off of it, doesn't mean they will always. What happens if the day comes where WoW is actually dying? Live has 400k active players and vanilla has lets say 3k or less....do you really think Blizzard wont pull the plug if the upkeep is more than the profit? That they will lose money just to keep something going for a few fans?
Or do you think that just because one sub covers it all that Blizzard isn't going to keep track on how active both are? I'm sure they will have the numbers, merging both subs into one is just a good method of plausible deniability