Can you show me a quote where they said they actually pulled resources from the other games to work on WoW?
http://www.sk-gaming.com/content/24908-WoW_delayed_Starcraft2_for_a_year
I agree comparing specifically the ESO and WoW is silly. WoW, when started, was being developed by roughly the same team at Blizzard as all their other projects, so development hindrance was really to be expected. But it isn't totally irrelevant to compare like scenarios.
It all boils down to:
If the MMO (ESO) is immensely popular, will the company (Zenimax) decide to put more development cost into the MMO as opposed to the SP titles? Well, look to the past. It has only happened once really, but to say its impossible for it to happen again? Its not a long shot to suspect that it could be possible, but it is not very likely at all.
If Zenimax was willing to open a studio dedicated to multiplayer titles, then they have already shown that it is unlikely they would divert resources from SP to MMO. If they simply took a few of the Bethesda devs to start the MMO project, as Blizzard did with WoW, then the two scenarios would likely be more comparable.
This all, of course, also relies on the fact that the MMO would be a huge commercial success. I am certainly going to try out the MMO, but the announcement certainly met a surprisingly bitter reception. As unwarranted as they may be, for some reason I have my doubts this will touch the MMO giant WoW.