One could say that most games aren't designed to accommodate multiple people being able to join due to the nature of the game design (allies, extra assistance could break the game/make it too easy).
I definitely think games getting more complex has prohibited a simple drop-in mechanic from being more commonplace. It works in Secret of Mana because it's all relatively simple. You already have a second and third optional player character, so whether they're controlled by AI or another player is pretty arbitrary to the game design. Most games don't work that way, of course. That being said, why couldn't this have been done with Mass Effect? I know that game is very story-driven, but how fucking cool would that have been?
The irony is the Wii U probably did this better than anything; I hope they port over the co-op Mario platformers to Switch to go with Mario Kart (and did they NOT port Smash?) for you and the boy. Those are perfect co-op for gamers of all ages and skills.
He really doesn't care for Mario Kart. His coordination isn't quite there yet. He just runs into walls and gets frustrated. But yeah, I'm really counting on Nintendo releasing 3D World on the Switch, because I've been dying to play it.
Speaking of which, I'm relieved that he prefers Super Mario World to Super Mario Bros. 3. Just like dad!

BTW, a reminder that you gotta try Shovel Knight co-op with him if you haven't already. It's Mega Man meets Mana!
Awesome, I didn't even know there was a co-op mode in that. I played it on 3DS, but maybe I'll get it for Switch if he's interested.
I see developers these days talking about game play loops and player feedback systems. It just seems too sterile or as if they are trying to manufacture a new type of drug that will addict the player in a responsible and satisfactory way.
Maybe I'm misunderstanding, but I think it's natural for game developers to want to refine the process of making games. And the industry has basically segmented into several different ones at this point, each with their own pools of players. There's the "mainstream" market for AAA games, the "indie" market for simpler experiences, and the "mobile" market for dumpster diving modern roulette. It's common for people to be concerned that games are getting focus-tested to death, but well, sometimes it works. Valve uses TONS of focus-testing and their end products are super refined — you know, when they remember they can
release games instead of just selling them.
EDIT: Oh, you mean like mobile games, designed to drain every last $1 out of you, right? If so, I honestly don't spend time on those kinds of experiences. They're just not for me.
Plus every damn game these days is so overblown and massive that its refreshing to just hand the controller to the kids and actually play the game.

PLAY. THE. GAME.
https://www.youtube.com/watch?v=930H_ZRExbgI'm curious how your son likes playing with a controller. My little one is pretty overwhelmed by the controller since she's used to touch inputs at this point but she loves the art style of Rayman, so we give it a try from time to time.
Yeah, his very first gaming (and laptop) experiences made him want to touch things to make them happen. But he's seen me use a controller enough that he understands some games use touch and some games use controllers. His actual hands-on time with controllers is pretty minimal, so when he does play, he needs guidance doing common-yet-complex things with controllers like doing a running jump in Mario (hold run, hold right, AND jump at the right moment). Watching him struggle with that make me consider what makes sense to do in video games. That running-jumping thing feels very un-Nintendo these days

But in other respects, he's grasping new concepts incredibly fast, which is of course what kids are great at. We played through Monument Valley over the weekend (he loved it), and he would end up solving puzzles where I was stuck ("Here, dada. Let me.")