I agree that the classics are usually better, but the actual situation is that most of the new stuff is bad.
Allow me to speak strictly as a person who enjoys cartoons for their charm and artstyle. I've been a huge fan of older cartoons, stuff like Scooby Doo and Samurai Jack (this being my favorite) amaze me the more I watch them and I particularly grew up with Dexter's Lab and Ed Edd & Eddy, but regardless of nostalgia I have to say some of the new stuff are completely enjoyable for what they're worth. Despite the awful fanbase, I found Adventure Time to be completely charming, and while Regular Show and Gumball seem to try hard with the comedy and isn't as creative as Adventure Time I still enjoyed the overall artstyle and presentation. I do agree Ben 10 falls flat due to how much it caters to kids with the action, but I can see why it's popular amongst them. (And it definitely does a good job in keeping them intrigued)
For me, comedy isn't what defines cartoons. So while I found them to be hilarious back in the day, they do feel plain today. Even when watching Ed, Edd n Eddy, which I used to think was funny, I don't see myself laughing, but I appreciate it more. The charm, the artstyle, the animation, the cast, the visuals and the creativity behind the cartoons are what defines them, particularly the charm. I think Regular Show, Adventure Time and Gumball are strong in those departments. I'm not expecting to laugh, and I set my expectations low, but I would be lying if I said I don't feel like a kid again watching them.
It's not just cartoons either. Games like Warioware, Little King's Story, Rhythm Heaven and MediEvil are games I love especially because of the influence of charming cartoons.
I'm worried more about Disney. They seem to be heavily reliant on childish sitcoms and pop culture nowadays. I can see that young girls enjoy it, but it's pretty bad.