A theory is Marvel and DC don't want writers to juggle marriage along with crimefighting. Also those kid friendly books get cancelled. They ultimately don't get big enough to be the next generation of superheroes to replace the current status quo.
But... that's just silly. You have superhero families like the Fantastic Four remain a family - with marriage and kids - for over sixty years. If you want to do a version where that doesn't happen, you create an alternate universe version where, I don't know, Reed loses his mind and infects them all with a zombie virus after She-Hulk eats their children, or turn him into a supervillain or something...
The point is, there's a billion ways to have an outlet for that. "Wouldn't it be nice if we had a young, black Spider-man kid in our book?... Oh wait, I'm the writer. I can do that in this alternate universe" (before he proved popular enough to merge into the new one - which was almost the only point of the "end of the Ultimate universe" event entirely).
And the whole "DC hates marriage" argument (which was their initial excuse when their own writers complained about being blocked on a "gay marriage" storyline) never held water. Editorial screwed up hard on that, and many writers openly rebelled. Pretty sure Aquaman's marriage to Mera (though hilariously and briefly "interrupted") endured nonetheless.
And you have plenty of decent-selling, kid-led titles like "Champions" and "Teen Titans" ("Nova" just relaunched to decent sales, but I admit Richard Rider's return helped, and I honestly wish Ms. Marvel would sell better...)
But Marvel isn't just comics, and public exposure does wonders for a character's comic cred. I mean, let's not pretend that before the movies that Guardians of the Galaxy - as good as it was - was anything more than a cult favorite comic series. The masses didn't know Groot or Drax or care about Star-Lord or Gamora, nor could they tell whether Rocket Raccoon was a comic character or a Star Fox character. Marvel gambled on some less-known characters, and the boost to their book and their comic status has been beyond noticeable.
The point is, the general public's impression of a character can shift radically based on the exposure they have outside of the very, VERY limited medium of comics. I mean, I could spend all day telling you how much fun the Great Lake Avengers are, but 90% of comic readers won't have picked up the book, no matter how gleeful and fun it is because characters like Doorman and Mr. Immortal don't resonate with them as strongly as Batman, Spider-man, or Iron Man.
I understand the sentiment you're trying to convey, and largely I agree with it, but DC permanently killing off the most widely beloved and culturally iconic supervillain of all time would be a bit rubbish. The Joker's too good a character to kill off forever and never return for as long as DC Comics exists.
Not for as long as DC exists... but for as long as it takes them to "reboot" (which seems to last only 10-15 years). DC keeps hitting the "reboot" button so often with entirely new, alternate universe takes that it's perfectly logical to have one universe where the Joker bites the big one and stays dead until the next reboot arrives.
I mean, the New 52 is not the old universe. We're having some characters from the old universe pop up here and there, but they're not the "same" people. Barbara Gordon isn't the old Oracle who founded Birds of Prey, Jean Paul Valley is up and alive again as Azrael, Wally West from the old universe just popped up but hasn't replaced the African-American version native to the New 52, etc., etc.
DC can absolutely kill off the Joker for "a" universe and be just fine until the next inevitable reboot.
