Okay, when did the last episode of Superfriends air? 1985.
What ELSE major in the world of DC happened in 1985? Well, the CRISIS ON INFINITE EARTHS, of course. After that DC didn't get a new animated universe until 1993 with the premiere of Batman TAS. It was also established in the new Scooby Doo movies that Scooby Doo took place in a DC Universe because Scooby Doo met Batman and Robin on two occasions.
After CRISIS ON INFINITE EARTHS, Scooby Doo was rebooted to a puppy in the late eighties in "A Pup Named Scooby Doo", sliding the old adventures forward into the 90s and What's New Scooby Doo into the 21st century. In other words, CRISIS is the reason Scooby Doo seems to the same age for all these years.
And, just hypothetically, the Max Fliecher Superman cartoons were part of the old DC Animated Multiverse, and the Superman of those old cartoons served the same purpose as Earth-2 Superman, eventually destroying whatever animated equivalent of the Anti-Monitor might have existed.