OK, since we are here anyway, here are my arguments about why it makes sense to NOT put effort into vanilla for quite some time. I don't expect everyone to agree but if you disagree at least we can talk about a specific item or items.
1. Doing vanilla right is a lot of work. We've been over this before, in sum - what Nost guys did is not at all what Blizzard should do, Blizzard's task is much more difficult. If Blizzard just do what Nost guys did, it won't be vanilla - as in, it won't be vanilla enough for the nostalgy.
2. Nost's numbers were impressive (and other private server numbers are impressive, too) - but a lot of that is probably because there was no pay. The numbers for a F2P title are always a magnitude greater than for a paid-for title. Yes, if Blizzard do vanilla themselves, that will attract more people than Nost / other private servers in that many people will just never play on anything other than official servers (several important reasons) and with Blizzard they might come and try. But there will be a big factor going in the other direction - having to pay. Even bundling that with the existing sub as an additional free service means pay - and that's going to reduce the uptake significantly.
3. Bringing vanilla servers not just requires a lot of work on the part of Blizzard (see point 1), it is pretty sophisticated work. We aren't talking about artists, we are talking about devs - and we need skilled devs. These skilled devs could absolutely work on Legion instead and since the amount of work for vanilla is big, the opportunity cost is big as well - the choice is to either have vanilla servers or have an extra big patch for Legion or a big advance (like 4-5 months) on the next expansion.
4. Finally, Blizzard aren't in good standing with Legion at all. They are losing players bigtime, this shows on all proxies. Legion so far failed to recover from the disaster of WoD, there was some uptick of goodwill / players at launch but that quickly ended and players are quitting again - and fast, seemingly as fast as in WoD. I wouldn't be surprised if Legion loses another 2-3 million players if things continue go that way (and there is a pretty good chance that they will, they need a miracle in 7.2 / 7.3 to stop that, and miracles are rare). It is just not the right time to divert resources.
Hope this is at least clear.