I agree completely! Once upon a time, there were great voice actors on cartoon shows. In addition to the ones you mentioned, I'd add June Foray, Paul Frees, Bill Scott, Thurl Ravenscroft, Don Messick, Paul Winchell, John Fiedler... all terrific talents. Nowadays, other than Edie McClurg and Tress MacNeille (and possibly Gilbert Gottfried), there don't seem to be a lot of professional adult voice actors in TV cartoons these days.

So what happened? My guess: Most of the budget for cartoon shows these days is spent on whiz-bang animation rather than on voices. Only animated movies have the budgets to get both great animation and great voices, and the movies usually hire famous movie actors to do the voices anyhow.

As a result, professional voice actors who might in previous decades have done cartoons find it more lucrative to do commercials. Kids, as you suggest, work cheap, and perhaps it's assumed that kids relate better to voices that sound like theirs.

BTW, if you want to blame someone for the kids-voicing-kids phenomenon, may I suggest... Charles Schulz. That's right, when the "Peanuts Christmas Special" was being made, Schulz insisted that only children could voice his characters. Considering the animation quality of said special, I can only assume the total production budget was somewhere in three-figure range. (None of which means I don't still love that special.)

Chiming in on an area of interest...

I think there's plenty of modern adult voice artists that do a good job---Tress MacNeille, Frank Welker, Maurice LaMarche, Rob Paulsen, and Phil LaMarr among others.

However, I think it's the subject matter of modern kids cartoons as a factor---unlike the old days (when cartoon characters tended to be adults, like Popeye, Fred Flintstone, Bugs Bunny, etc.), current cartoon characters are almost always children or teenagers. The only adult cartoon characters these days tend to be either non-kid-oriented shows like the Simpsons, or some of the superhero shows (though even these tend to focus more on teenage or child superheroes these days). Unfortunately, the slapstick cartoons we grew up on (like "Looney Tunes") don't seem to be popular at all these days (aside from perhaps "SpongeBob"---which is probably also the one kids' cartoon that also revolves around adult characters who aren't superheroes, SpongeBob's childish personality aside. Hmm...wonder if those are reasons it's as popular as it is... ;-) ).

With most cartoons seeming to revolve around "kids going to school" scenarios (like the deplorable "Recess"), not surprised that most kids' show character voices sound rather similar. Though if I were a kid, I'd think I'd want more variety than just shows about kids my age going to school... (shrug)