The legend has it that ABC was threatening to send Scooby-Doo to the ratings kennel after Scooby's All-Stars (formerly Scooby's All-Star Laff-a-Lympics) had seen a steep decline in its 2nd season. Desperate to keep its last franchise of the 60's a relevant player, Hanna-Barbera added to the Doo family by introducing Scooby's nephew, Scrappy-Doo, as a 10th anniversary present to Scooby and his fans.
Yes, Scrappy has his haters, and it's clear the writers of 2002's "Scooby-Doo" feature film played to those particular folks by making Scrappy the villain of the piece, a move decried in some corners as being mean spirited and disrespectful, given that Scrappy was exactly the ratings boost the franchise needed.
Scrappy's entrance made Mystery Inc. a sextet instead of a quintet, but that wouldn't last very long, and there would be more changes rather quickly. Actress Pat Stevens, who succeeded Nicole Jaffe as Velma 3 years earlier, left the show after recording 11 episodes, and was replaced by Marla Frumkin. Veteran actor-filmmaker Len Weinrib (ex-Magic Mongo) was the original voice of Scrappy, as you'll hear in the show open below, but was replaced the following season by Scooby's portrayer, Don Messick. Weinrib's on-again, off-again relationship with H-B was off again, and would stay off for a few years.
Speaking of the open, here it is, uploaded by Hewey1972:
The familiar, but still tired, mystery format was discarded after 15 episodes, and Velma, Fred, & Daphne were sent away for a lengthy vacation, putting the spotlight on Shaggy, Scooby, & Scrappy for the next three seasons. Daphne would rejoin them for the 1983 season (The New Scooby-Doo Mysteries), in an effort to bring the traditional mysteries back into vogue after three years of slapstick comedy. Was any real collateral damage done? It's hard to say, really. People will have their opinions. Scrappy brought a renewed energy and an uptick in ratings, but whomever it was that decided that Shaggy and the Doos would be a comedy team on the order of the Three Stooges or Marx Brothers nearly ruined the franchise, not Scrappy.