There's always a generation gap for each decade.
1930s: People who grew up on the 20s shorts probably complained about the newer cartoons bringing color.
1940s: People who grew up on the 30s shorts probably complained about the cartoons being less about singing and more slapstick and violence.
1950s: People who grew up on the 40s shorts probably hated the fact that it started to become watered down and become less funny.
1960s: People who grew up on the 50s shorts probably complained that the animation was sorely lacking and flat designed.
1970s: People who grew up on the 60s cartoon shows probably hate the fact that the Mystery Inc. phase was in effect along with boring cheesy rehashes.
1980s: People who grew up on the 70s cartoons hated the babyfied/Smurf era where every studio has cashed in within the toy markets, along with Korean studios taking over, and what are those weird eyed shows from Japan?!
1990s: People who grew up on the 80s shows probably hated the more creative humor type shows along with serious shows with superheroes. Yellow people? A dumb cat and crazed chihuahua? Two Warner Brothers and the Warner Sister? An all animation channel?!! Teen blocks taking over along with the FCC mandates of the late 90s?? Tartakovsky influenced flat designs? Super saiyans?
2000s: People who grew up with the 90s shows complained about the shows being for kids, childish humor, uninspired animated radio. Pokemon. Oh, and a sponge who lives in a pineapple under the sea taking over.
2010s: People who grew up on the 2000s shows probably complained about the Adventure Time led age, reboot after reboot, less action, more weird humor and inclusive shows made from different races and genders.
So you see, in the end there will be someone else making a thread just like this one a decade from now, and people will complain about something that never belonged to them.