The pseudo-experts have really been coming out of the wood works, haven't they?
He's not using the term "reverse engineer" correctly. I don't even know what he's talking about. Realistically, they would start from the source code of the last release build pre-2.0 for both client and server. This would involve using the old tools, but a company like Blizzard surely has periodic backups to pull from.
That build should be mostly bug free because it's the release build that was on live for months. For bugs that still exist in that build that were fixed later, they can find them in the bug tracker and check commits in the revision control system around the date the bug was closed. That tells them exactly how the bug was fixed and bypasses 99% of the work that goes in to bug fixing. QA costs would be considerably lower because they're essentially just fixing bugs and turning content on/off on the server side.
The vanilla server was significantly less complicated than the cross-realm and battle.net integrated beast that WoW is today. It was simply one self-contained realm per server. The hardware costs would be negligible (it is 10 year old software) and Blizzard already runs server centers. Set up would take some time, but Blizzard is very good at this, it's not as if they're throwing a "Networking for Dummies!" book at a junior programmer.
Sometimes people bring up customer service costs. The cost/player might actually be less than live because there isn't an in-game store, achievements, etc. The game was just much smaller than with far fewer things to go wrong. They already run customer service and they're used to constantly changing staffing requirements as subscriptions rise/fall.
As for marketing... why would you think about marketing when determining a project's viability? No one's saying "Oh this game would be great, but it's particularly hard to market so marketing would cost too much." This really depends on the chosen business model.
The argument that resources will need to be moved from live WoW is also stupid. Blizzard is a massive company with thousands of employees. Only people within the company familiar with its organization and current projects/teams knows where the talent would come from. Since Legion is their next title launching, it's probably the least likely place they'd shift resources from.
I'm really sick of hearing pseudo-experts talk about the financial feasibility of legacy servers. Blizzard has huge teams work on projects for years without deadlines, and for every project you hear about getting canceled (Titan) there are many more that they never announced. I guarantee you that WoW has had large features developed and canceled that rival the cost of legacy servers.
The point is that game development is difficult and expensive. Projects frequently just don't work out and end up canceled. Developing legacy servers is not the same as developing a game, it's much cheaper with far less risk. Financial feasibility is not an issue. Period.
All of you MBAs with a background in software engineering and inside knowledge of Blizzard's finances, development processes, organization, and the World of Warcraft source code: please shut up.