This is what I think happened to the Scooby-Doo gang after the TV series ended.
The TV series was the story of four high-school students and one dog. Most parts of the story are set during their summer break. The four kids graduated from school and grew up. The dog grew old an a veterinarian had to make him go to sleep. Shaggy missed his friend and sometimes wanted to hear him again, but he knew there was no such thing as ghosts.
Daphne married a famous person. Velma earned a reputation as a genius. Fred became a rich man and avoided controversy. Shaggy became a good man.