Hands down, the best case against Real Monsters in Scooby Doo is presented by Chris Sims who advances the idea that the Scooby Gang should be viewed as avatars of skeptical/progressive thinking in a world of superstitious authority figures.
In addition to that, Chris (and many others) argue that having real monsters in the show subverts original premise of the show in a way that makes it unwatchable. I disagree with that for several reasons.
- For kids originally exposed to the show, the monsters were real. Constantly relying on the gimmick of a person under a mask just caused kids to stopped taking the show seriously.
- There is the perception that the introduction of real monsters into the show is a modern conceit, but that's false as the first real monster was introduced in 1970 in the That's Snow Ghost episode.
- There are also merits to portraying the gang as heroes who aren't afraid to confront unknown challenges of possibly supernatural proportions.
- Also, it could be argued that the world we are seeing plays by different rules than ours.
- He mentions technology being different in the show (working robots, electric monsters from just car batteries, transparent glider skies, ect...)
- Scooby can talk and understand complex human sentences
- When in monster guise, the villains behave irrationally, as if they are not in possession of their own wits. (Like the Werewolf villain who is befuddled by Scooby and Shaggy pretending to be barbers:
Also, in the The New Scooby Doo Movies, the gang teams up with both Batman and Robin, Josie and the Pussycats, The Addams Family and Jeannie (and her apprentice genie Baboo.) While it could be argued that the Batman and Robin of this world are from the 60's television world (where there was no magic), the presence of Josie and the Pussycats, The Addams Family (somewhat) and Jeannie (definitely) suggest magic exists in this world.
The biggest surge in real monsters came with the advent of the direct to video animated movies with Scooby Doo and The Goblin King being probably the most fantasy tinged entry in the series. However, more recently there has been an effort by the newer movies to present all monstrous threats as guys in a costume. It appears that the last appearance of a real monster in the universe was Annuki in the excellent Scooby-Doo! Mystery Incorporated.
Yet, in order to remove all real monsters, the movies have had to stretch physics to herculean contortions to explain how the villains are able to do the things they do. (Scooby Doo and Kiss for example uses dream sequences and super advanced technology to explain its plot.) This avoidance of supernatural elements in the show has gotten so bad that even Blue Falcon has been demoted to just a fictional comic book character.
In a way, it reminds me of the eye rolling lengths that the Marvel Cinematic Universe has gone to keep explain away Asgardian magical items as quantum powered alien technology.
The opponents of real monsters believe that the creatures ruin the show/movies for them because it perverts the original premise. I would argue it no more ruins the show than having magic work in the Batman universe. The Golden Age Batman universe wasn't filled with magic, but the modern one certainly is. Where is the hate for magic using heroes and villains in the DC Universe or Batman Adventures? There is none.
At the end of the day, a lot of the hate is just people who feel like their version of a show has been corrupted to something that no longer represents their childhood memories. That's too bad, because they are missing out on a fun element of the show.