I was initially going to ask the question of Why was Scrappy-Doo introduced to the Scooby-Doo series, but that seems to be answered succinctly by the below Wikipedia paragraph:
In 1979, Scooby's tiny nephew Scrappy-Doo was added to both the series and the billing, in an attempt to boost Scooby-Doo's slipping ratings. The 1979–80 episodes, aired under the new title Scooby-Doo and Scrappy-Doo as an independent half-hour show, succeeded in regenerating interest in the show.
However on skimming the article, I can't see anything about a negative fan reaction.
When did that happen?