However, if Legion fails, the WoW subscription numbers are going to tank and this will raise a few questions about the direction of WoW and it may provide the incentive to create legacy realms.
MMOs tend to be on a decline, they peak at some point and only pickup on new releases especially when you consider this game being on top of the subscriber chart, breaking most records in sales on release and subs for quite a long time (it's highest being during WotLK iirc?). I wouldn't expect it to get any better in Legion, it'll peak at the start but declined fast towards mid-life.
The current ~10k subs to Nostalrius may have a reason to play Vanilla (It was fun back in the day, I agree) but that doesn't mean that if Legion fails, people will resort to Vanilla servers.
Jagex did so and now there are two entirely different games of RuneScape, each being updated regularly.
The costs with creating and maintaining these legacy realms are also not expensive at all, considering a team of hobbyists were able to do it.
I don't know Jagex well enough to make an informed opinion, but I know software and diverging software paths are quite difficult to maintain *especially* when you have 8 years worth of bugs and fixes that ought to be in that vanilla realm. While a team of hobbyist quite easily could settle for a working vanilla instance disregarding the entire fixes that went through these 8 years (9 assuming pre 1.1?), blizzard would not.
Blizzard would rather tell its subscribers "we know what you want" than give them the honest truth about their profit margin concerns.
Money talks, and its voice is getting louder.
They've made and still are making drastic mistakes in their design decisions, which is costing them a lot of subs. Legion has the very same flaw that WoD had, they've simply renamed it "Order Halls".
I hope Legion succeeds.
