I have a philosophical disagreement with you but I can't quite pin it down. The best I can do is compare The Hobbit, a children's fairy tale, to Game of Thrones, a show meant for "adults." Game of Thrones is considered more mature in the eyes of most people due to its nudity, profanity, and violence, and yet the presence of those very things make it immature. The Hobbit, on the other hand, promotes that which is good, not cynicism and sin, and thus, despite being written for children, will always be more mature than Game of Thrones.
Though I do prefer the YouTube Mickey - that was very nicely said.
I think there's a lot of nuance in the idea that "mature audiences" like the new Mickey more. It's not so much that it's about maturity, but rather us grown-ups are a little bored with the Walt-era quest for realism in animation.
Walt was so determined for the world to see his vision that animation was a legitimate art form, that he always was pushing for more and more techniques that put his films on equal footing with live action and/or painted masterpieces. (Controversial opinion: Walt would have loved Jon Faverau's Lion King.) But post-WWII saw animation being produced more cheaply, especially once television entered the mix. And Walt's interests and obsessions flew elsewhere.
For the purposes of this conversation, I'm calling Mickey from 1940-2010 "Realistic Mickey." There's consistency in the placement of Mickey's nose, the appearance of volume in his torso, eyeballs instead of black pies, etc. As many of us know, there was even a brief period in the 1950's when they tried to get his ears to take up 3D space realistically.
But Classic Mickey (1928-1939) was the one that "started it all" and is the icon.
And these days, "mature" audiences find more visual interest in the playful deconstruction of the icon. He's a toon! A graphic shape made of graphic shapes. It's not that kids wouldn't find the jokes in Bad Ear Day funny, it's that adults also get that there's a history to the iconography of Mickey's ears.
In fact, the first few of the YouTube Mickey shorts practically read like a list of riffs based on classic character iconography:
Why did classic Mickey wear no shirt but Donald wear no bottom?
No Service
Why did Goofy's hat always look like a cartoon bone?
Ghoul Friend
Again with the ears - why are they the "decorative" item on Mickey?
Hats Enough
Why are Pluto and Goofy both dogs?
Dog Show
It's not so much that you have to be "mature" to appreciate the humor. It's that kids wouldn't know that the "Realistic Mickey" can't go there. There's no way for
this Mickey to wink at the audience and acknowledge that he's a graphic design icon because this version has had the graphic design sucked out of him in exchange for realism.