Of course they want it to fail. It makes about 90% of the people working on wow meaningless and redundant if it succeeds and bfa tanks
The thing is though that it doesn't need to mean that. I know it's hard for game development companies to wrap their heads around it, but sometimes their best intentions are based on bad decisions.
If World of Warcraft Classic turns out to be a smashing success, then it would be in their best interest to accept that their current direction for the game (aka BFA) was the result of bad decisions made over the years. If that was the case, then it would be prudent for them to shift their resources from making expansions for current World of Warcraft, and instead start making expansions for World of Warcraft Classic based on the old game design principals that start off as the first expansion to the Classic platform, and not ever branch over to any of the existing expansions.
Unfortunately, most MMO companies are too proud to admit when they were wrong. Sony Online Entertainment (now Daybreak Games) is a shining example of this failure. The Time Locked Progression (TLP) servers was their version of Classic, and it has been a resounding success for many years now, but they never took the hint. Instead of creating new content for TLP, they simply let the TLP servers go up through the existing expansions. This has lead to a recurring cycle of people playing the game for the first few expansions, and then stop playing. It's not that they don't like the game. It's because they don't like the way EverQuest changed over the years, and they don't want to play through those expansions. They didn't want to play through them the first time, and they certainly don't want to play through them again.
Hopefully at some point one of these MMORPG developers will figure out that its ok to start over and take a different path with their game, but who knows if or even when that will happen.