They've already mentioned that time dedicated to focusing on making classic servers a thing and maintaining them would mean less attention/man power on the current more important stuff. I'm pretty sure the company developing the game for the past decade has a pretty good idea of how much time and effort this would be to juggle along side the current iteration of WoW.
People make the argument that Nostalrius did a pretty good job of maintaining a Vanilla server. But they too struggled and it had it's problems too, and they still needed a full team of people giving up their free time just to keep this old snapshot of the game running. People boast about 800k accounts were on that vanilla realm, but how only 150k of them were even active, a very small portion in comparison. I imagine that most of those 650k inactive accounts were people who went to try the vanilla experience and realized it wasn't as good as they remembered.
This whole topic is ridiculous.