Sure, it's an evolution, but it's the most knuckleheaded version. It's 'escalation as development'. If this guy had the Joker kill five people, I'll have him kill ten people, and the next bloke has him kill twenty people, five nuns and a puppy dog. It's dimwitted storytelling. Rather than telling BETTER stories, or hell, even GOOD stories, they're just upping the bodycount... like that's some sort of barometer of quality.
It's the sort of logic that says 'Friday the 13th Part VIII: Jason Takes Manhattan' is a better film than 'Psycho' because only two people get killed in 'Psycho'.

If DC is emulating Thomas Harris, it's doing so in an adolescent manner. It's all about execution, not content.
It reads like some surly 14 year old metalhead saw 'Silence of the Lambs' and said, "Hey, let's do a Joker story, but we'll make him all fucked up an' shit. He can, like, get some dude to tear his own face off an' shit and then staple it back on because he's so hardcore, and then he fucks up Batman an' shit and Batman's all like, 'Oh, man, the Joker is so fuckin' hardcore, I don't know if even I can stop him an' shit.'"
Dan DiDio's attitude that everything needs to be about failure and trauma is an adolescent conceit.

When you become a teenager, because you have elements of childhood and adulthood in your make-up, you want to emphasize the latter. You want to make yourself look grown-up. But because you're still also partly a child, and you have no experience of actually being a adult, you have no idea how to do that. 
So what you do is take what you DO know - the things of childhood - and turn them upside down. Because adulthood is the opposite of childhood, right? So you take the things you loved as a kid and say, "That shit is childish. I hate that shit." You reject things like fun, laughter, optimism, altruism and fantasy, and embrace bleakness, misery, pessimism, cynicism, and nihilsm, which you consider to be 'realistic'. "Man, that shit is bullshit. In the real world, Batman would be all fucked up and psycho and shit. Not that happy, smiling bullshit. This is the real world, man. Nobody just walks away from that shit."
That's why teenagers drift away from Superman, towards Batman. Because Superman is bullshit. He's too perfect.
But Batman (Billionaire, Scientist, Master Detective, Martial Artist in a Halloween Costume fighting an evil supergenius clown) is 'realistic'. And they'll say that world 'realistic' like (a) it's the most important thing in the world, and (b) like they actually mean it.
And hell, even as a teenager, if you still like fun and fantasy, your increased social awareness means that you're insecure that someone might notice and make fun of you for it. "What, you're still into that kiddie shit? What are you, eight?"
That's why almost all the promotional material during the 80's was about how 'Comics Aren't For Kids Anymore'. Like that was something to be proud of. "We've taken all the kids' toys away from them and that's awesome."

An adult, on the other hand, doesn't need to reaffirm his or her adulthood. They do that every day by going to work and budgeting and paying bills  and listening to their teenage kids tell them, "Hey, you don't understand, man. It's all misery and futility."
So an adult can sit with their children and everyone can enjoy watching a cartoon rabbit get the better of a bald hunter with a shotgun and a speech impediment, while their teenage kid sits grumpily in the background and says, "That's bullshit. In the real world, a hunter would straight up kill a rabbit."

Hopefully, they'll grow out of it. The sad thing is, some of them never do. The even sadder thing is, some of them become heads of one of the biggest comics publishers on the planet.

DC Fans aren't unitary.
There are a LOT of fans of DC's characters and universe who feel that the company's product is actively geared to drive them away. That's my point. DC has all the good toys, but is only interested in marketing to those who share their extremely narrow viewpoint of what sort of comics they should be publishing; the so-called 'hardcore fanbase'. All they need to do is stop catering to this selfish minority at the exclusion of everyone else.
And a big part of that is removing the stick up their butt that they have about how they somehow have to 'fix' their characters so they're 'darker' or more 'realistic'. They don't. People LOVE Superman, Batman, Wonder Woman. They don't NEED fixing.

What disturbs me most about Didio is that he always seems to be laughing hysterically...at everything. It's almost as if he got a whiff of the Joker's laughing gas and it never quite wore off. 

I still don't think it's all about realism, spurious or otherwise.

Kids themselves play one-upmanship games about what is or isn't fitting to read. I can remember being in some middle-school grade-- I'll say seventh-- and that when I made some remark about a MIGHTY MOUSE comic I'd read, I immediately got the "only little kids read that junk" reaction from a peer.

You might assume that this proves your case, since I suppose I and my peer would have been about 12. But I descried the same reaction from adults about anything too cartoon-y.  So I can't buy the idea that all adults can sincerely enjoy watching Bugs Bunny trounce Elmer Fudd. I think that in the past twenty-something years, it's become a little more acceptable for adults to enjoy such simple pleasures; otherwise, sites like "Bleeding Cool" would never be doing articles on favorite cartoons and comics. But a lot of adults are still impatient with simple pleasures, and won't watch Bugs Bunny except if they're enjoying their kids' pleasure in the cartoons.

Again, this isn't just taste; it's a hierarchy based on what is perceived to be rigorous, demanding, and adult. Around the time of Superman's 50th anniversary, some of you on this board may remember that TIME MAGAZINE did a cover-piece announcing "He's 50!" Around the same time, a TV commercial for some other magazine-- I no longer remember which-- featured a spokesman who held up the TIME Superman-cover, repeated the words, "He's 50," and then said, "Who cares?" The rhetorical point was to make TIME look foolish and un-adult for devoting a major story to a kid's comic book-icon, and a lot of adults still respond to that sentiment, not just sulky teenagers.

(Even if I didn't remember that the commercial was dated by Superman's 50th, you'd know if must've happened a really long time ago-- who advertises magazines on TV any more?)

The demands that come from the age-hierarchy aren't rooted only in teens being insecure about the image they project. Other little kids dis each other about their "kiddie" tastes, and adults look askance at other adults for just about every breach of "adult decorum." You're giving me the impression that either you've never personally seen this kind of put-down from anyone but teenagers, or that you've seen it and have somehow considered it irrelevant. Unless you have a third alternative--?

A lot of people have a sentimental liking for the great DC characters, but those people won't support them.  Recently THE BEAT did a piece talking about how many women on Facebook would "like" comics characters, as evidence of How Things Have Changed. Detractors pointed out that it costs nothing to say that you like something on Facebook, and such stats prove nothing about whether or not these people actually purchase comics.

I don't even think the "hardcore fanbase" is unitary in the sense you've described it. I think that these are the direct descendants of the impulse buyers of the mainstream days; they get interested by Big Events in exactly the same way an earlier generation got interested in why the Flash had grown a super-sized skull, or how the Thing would fare in a fight with the Hulk.

A lot of these "Big Events" are meretricious, but sometimes they work. I hated the concept of EMERALD DAWN and thought it was badly written. But the event drew people's attention back to Green Lantern and made him a major DC player again. I'd like it if, as you claim, the only way to success was through "good storytelling." But it ain't necessarily so.

An additional question:

Is the "power fantasy" associated with superheroes limited to that genre?

And do most readers really "grow out of" enjoying such fantasies?