Well they have 3 choices:
Redesign classes and leave Raids alone making them faceroll, given that the hard part of old raids was mostly class limitations more than boss design. People QQ that it's "not vanilla".
Leave everything as is causing the inevitable QQ that "I can't play my favorite class 'cos it sucks dick".
Redesign raids + classes, then get accused of funneling resources into Vanilla servers instead of new content when there's an inevitable shit tier.
Personally I want to play Vanilla (I joined during TBC), and I'd be fine with it left untouched (warts and all) but honestly, this is going to be a huge headache for Blizzard I think.