I've had this theory for a while now. It sounds kind of crazy, I know, but it's really kind of the only explanation for certain business moves that DC (and its parent company Warner Brothers) makes. My theory is that they have some sort of mystical engine hidden away in the basement of the DC offices, a vast and terrible engine performing some sort of arcane function that is absolutely vital to the company. And this engine is fueled entirely by Alan Moore's fury. So every once in a while, DC has to just go out and do something for the sole purpose of infuriating Alan Moore.
That's really the only explanation I can come up with for this. There's certainly no creative reason for it; nothing Johns is saying makes a lick of sense as a plausible piece of story logic. This quote: “If you’re going to have a conflict between optimism and pessimism, you need to have someone who represents a cynical view of life and also has the ability to affect this," said Johns. "I know it’s crazy but he felt like the right character to use," highlights the fact that Johns apparently either didn't read or didn't understand a comic that was all about Doctor Manhattan rediscovering the tiny miracles that are all around us and setting out to do something wondrous and transcendent instead of being imprisoned by cynicism and despair. (Well, insofar as 'Watchmen' was "all about" any particular thing, because it really did have a multiplicity of ideas, but certainly that's what Doctor Manhattan's arc was about.)
And it's not a sensible business decision, either. Setting aside the plan to once again urinate on the smoldering ashes of your relationship with the man who penned all the best sellers in your back catalog, it's not as if the world is really clamoring for more 'Watchmen' written by people who aren't Alan Moore. If 'Before Watchmen' settled one thing, it was that. Nobody is interested in 'Watchmen' for the characters, they are interested in 'Watchmen' for the writing, which is the one thing that DC has, largely through their own actions, placed entirely and permanently beyond their reach.
(Perhaps that's actually part of the problem--DC, like Marvel, is predicated upon the notion that creators are valuable but ultimately interchangeable, and that the true value is in finding intellectual properties that they can continue to sell no matter who is behind the metaphorical camera. Having a series like 'Watchmen' that they cannot exploit in any meaningful sense without respecting Alan Moore's talent and autonomy is like a poison pill for them.)
Honestly, given Johns' tone in the interview, the decision seems to be entirely motivated by petty spite. It's as if Johns is saying, "Hey, comics aren't fun at all, they're joyless and despondent and filled with unlikable anti-heroes! Who could possibly be responsible for that? Well, I told everyone back in 2005 that it was all Marv Wolfman's fault, and yet somehow things still haven't gotten any better in the ensuing eleven years? I know! I'll write a comic that blames Alan Moore!" And meanwhile, the guy who's been working at DC for the last sixteen years and who's been their Chief Creative Officer for the last six somehow skates. (Who could that gentleman be? I wonder...)
Honestly, I really hope that engine is doing something useful. Because I've pretty much given up on any hope of getting good comics out of the whole deal.