-snip-
... Honestly, my man. Everything you just said is so incredibly backwards. How a story is told does not a television genre make. There isn't a flashback genre or a narration genre. Genre describes the content of the show (as well as "form" and "style" though slice of life implies only that you are being shown a person's average day.) AND since everything is in fact, a slice of somebody's life, it is not a suitable reference by which to categorize a show.
Is it a pejorative? Maybe. I use it as one for sure. As a pejorative it comes to mean, as previously stated, something incredibly mundane and generally without conflict, with no direction, etc. and I have heard it used as such many a time.
Horror is a genre. When someone says something is horror you can expect to be scared, with slice of life you get no further information about the subject.
And also... What? A situtational comedy achieves everything because it is just that. Can you imagine The Office, but instead we just follow Jim around all day, he works, he gets groceries, he goes home, he kisses Pam, he falls asleep. That's slice of life. I mean... the only way to really depict it is as akin to soap operas.
Also, Hey Arnold could be very serious. Exploring the serious through comedy does not at all lessen the impact of a message.
Listen. At this point, I've become too lost to really hold any shows up as an example, but I'm an entirely confused as to your definition of slice of life, and it's context within the literary world.
Here's the definition, or one of many very similar ones: "A narrative technique in which a seemingly arbitrary sequences of events in a character's life are presented, often lacking plot development, conflict and exposition, and often having an open ending." -- Wikipedia
Nothing in that definition is good storytelling. Nothing in there makes me care or imparts any meaningful knowledge to me, and is thus, fucking worthless in the most literary sense.
AND, let's take it a step further--because Wikipedia provides not one, but two really good definitions. The other one is, "A very realistic depiction of everyday experiences in art and entertainment." Which is whatever. That part, I don't like either--as you can't really achieve that (and you shouldn't want to frankly.) The point is, I don't know what the fuck we're talking about any more. Literally, no idea. My assertion here is that "Slice of Life" is an experimental form of literature in which, nothing is said and no story is told in an attempt to impart some meaning from chance or lethargic non-action. Therefore, any show conforming to that is inherently worse off and has to work a lot harder to keep me interested. You can argue that is indeed what makes for bad writing.
To further address your last point, what you're describing is essentially the pseudo-genre of Literary Fiction. Another extremely bullshit idea that's stemmed from academia's poor handling of creative writing subjects. There is nothing more boring, dull, and downright bad than where our mundane reality reigns supreme and average people follow its inherent rules while attempting to solve ordinary problems.
I no longer have really much to say about any one cartoon in particular at this point. Steven Universe is a good show. Mostly because of it's world building. BUT, slice of life in television, and cartoons in particular are--to me--what makes television a vast wasteland.