Submitted for your consideration: Fred and Daphne never existed; they are "Tyler Durden" (Fight Club spoiler) fantasy personalities created by a drug-addled Shaggy and Velma. Shaggy, the dimwitted gangly slacker stoner, fantasizes a friend who is a natural leader with classic good looks, a strong physical build, and naturally brilliant. Velma, the dumpy outcast smart girl who is always overlooked imagines a best friend who is a beautiful debutante who can always rely on others coming to her rescue.
Watch the old episodes closely - Fred and Daphne never actually *do* anything, it's always Shaggy and Velma who get stuff done.
Further proof can be seen in their clothing choices - Shaggy and Velma dress in regular clothes. Fred wears an Ascot, Daphne dresses in a purple mini-dress with white go-go boots. It's like Shaggy and Velma are buying clothes from Goodwill, and Fred and Daphne are almost *cosplaying*.
"Scooby-Doo, Where Are You?" is where it all begins. The IRL story is that Shaggy convinces Velma (a very straight-laced "Good Girl" to drop out of school and drive around the country with him. They spend all their time in the back of the van getting stoned and discussing "Mysteries" like ghosts, and Velma blows holes in all of Shaggy's stoner-talk with actual facts and knowledge. Her standard end-of-argument statement is always along the lines of "See, it's not a GHOST, it's THE MAN! It all comes down to MONEY! It's a huge conspiracy where they want you to believe in all this hocus-pocus but it all comes down to an old dude in a mask trying to get more money".
Oh, but it goes deeper. "The New Scooby-Doo Movies" where they teamed up with various random "Celebrities" was just when their delusions were fueled by the acquisition of a little black-and-white TV set. Don Knotts, Laurel & Hardy, The Three Stooges, Batman and Robin, Dick Van Dyke - all "Special Gust Stars" that one would see in plentiful re-runs on their little black-and-white portable TV set in the 1970's.
Then things get sad when you consider that in the 80's it was just Shaggy, Scooby and Scrappy-Doo. Velma obviously had either a miscarriage or an abortion which resulted in their break-up. Unable to summon his delusion-friends any more, it's just Shaggy and Scooby now... and a Puppy that Shaggy projects all his hopes and dreams that he had for his own child some day. He'll be BRAVE! He'll be FUNNY! He'll look just like Velma and... No, not like Velma. Velma is gone. Have to remember that. Have to keep my head straight. My Son... he... he'll look just like Scooby. And so he goes to investigate all those "Mysteries" that he had explored with Velma, only now? Now they're REAL. He can no longer see through simple costumes! His reality is so fractured now that he sees Vampires, Werewolves and Ghosts EVERYWHERE! And he's frightened... very, very frightened...
AND THEN IT GETS WEIRDER!!! Because the *next* season features Shaggy, Scooby, Scrappy... and DAPHNE?!?!? Shaggy is grasping, trying to remember any time in his life where he felt happiness or joy and can only remember those times with Velma... only it's not Velma he sees in his memories, it's Velma's fractured-psyche Goddess Avatar! The long-haired beauty in the flamboyant purple dress and go-go boots! And she's there for him!
I think that those final seasons, the ones before "The 13 Ghosts Of Scooby-Doo", when watched in the proper order are a detailed account of Shaggy's suicide by heroine overdose :/
Scooby... Dooby... doooooooo... Where are... yooooooou... we've got some... work... to do... nowwwwwwwww...
(Reposted from my own LJ account to combine afterthoughts I have had on the subject and have a singular post that I can point the rest of the internet towards. I *think* I may be the first person to suggest this theory)