Scrappy Doo: Honestly, I think the reason Scrappy kinda overshadowed Fred, Daphne and even Velma to where they were pushed aside was because they were well... kinda lacking in personality at the time, at least compartively. Scooby and Shaggy kept themselves afloat through comedy, but the others lacked enough writing 'meat' in them. Fred was generic leader, don't remember much what Daphne did (which is more my crud memory) and Velma was the smart one with the clumsy glasses bit.
Honestly, Scrappy's problem that he was the scapegoat for the bigger problems of lazy writing and executive decisions, hence why the franchise happily embraced him being this target, it was a convinent excuse. I am unsure what could be done since the alternate would mean given Fred, Daphne and Velma more characterization and well, what's easier for the staff to do? That and I have my fondness for "Boo Brothers", "Ghoul School" and even "Reluctant Werewolf" so I wouldn't want to get rid of those. Maybe phasing him out sooner could've worked, but not sure there.
It's honestly trickier here since it would require doing things that I'm not as certain the staff behind would do. The franchise was getting stale and they were looking for novelty. Retiring him sooner could've worked, but when actually? Because honestly, it just seemed like he was the inconvienent scapegoat for a bunch of writing problems. Shifting him sooner could've worked enough to avoid being hated on and the blame passed to someone else.
I do miss Scrappy alot and oof if I had the chance to write I have a bunch of choice words.