Why the newer cartoons are so much shittier can be summarized easily.
Technology. Making cartoons has never been easier and as you can expect more and more people have started making cartoons. Look at things like ToonBoom, Flash, Go Animate! There are literally dozens of options to speed up the process, but just because we have more access to this doesn't make 100% better toons.
Look at Pickles and Peanut. Hastily thrown together using flash animation and stock photo images from photoshop. Hell look at Ctrl-Alt-Del the animated series. Animated in flash and boy does it fucking show.
That is not to say flash is bad its just the one everyone uses and thus has more shits than gold. Look at Star Vs The forces of evil. The animation isn't that great, but it does have some witty stories at times.
Case and point. For every good show such as Gravity Falls, Rick and Morty, or Futurama we get tens and twenties of a dozen shitty shows that aren't all that great, but make stupid people happy by being "topical" or trying too. It also isn't new. Look at the 60s - 70s. They had the Osmands, Happy Days Gang, and Casper solving mysteries in fucking space. Mostly trying to build off the success of scooby-doo.
We also need to realize that cartoons from the 80s,90s, and 2000s that are still on today are starting ot suffer from seasonal rot. They are getting more and more worse. Look at the Fairly Odd Parents they added a fucking dog. The simpsons stayed on by doing god knows what. I know They wanted to add a new character but rejected the idea. Instead all they did was copy southpark by being "topical".
Did Life in Hell ever make it to film or was it always kept on paper as a concept?
He wanted to put them on the Tracy ulman show but changed his mind because he didn't want to give up copyright of his characters. Instead he created a family he didn't give two shits about. Hell at the end of the first season of the simpsons. Marge was suppose to shave her head and reveal bunny ears, but that idea was scrapped when the show got more and more popular.