Humans have several millennia of evolution "touching things" for tactile feedback. Mice on the other hand are a relative (and quite useful) hack compared to that. It requires training and a different mental dimensional model to use. To a child growing up with modern touch screens, a screen without touch is the aberration and "broken". Mice may not disappear entirely, but certainly more and more screens in our lives will support touch interactions, even on desktops and "non-mobile computers", simply because it requires less training and feels more natural ("touch the button" versus "move the mouse to the button and click" may not seem that different after decades of mouse usage, but are very different experiences to an untrained person). There's a sense already today that kids growing up with tablets find mice strange and annoying ("Why can't I just touch that?"), and it's easy to wonder watching today's kids if mice will last for more than a footnote in history and in increasingly small niches that require "headshot precision" like games and maybe CAD.

Dunno. But my niece and nephew are tablet/phone experts, and have never used a mouse. And they are 4 and 6.

If it comforts you, I used an upside-down SNES gamepad for a few years before ever touching a mouse. And today I use my mouse far more than any game controllers.

I'm more curious why it was upside down.

I just grabbed one and starting using it upside-down. By the time anyone told me it was wrong, it already felt more natural to use it that way.

I think that just means new computing devices have expanded the market, not replaced old devices. Some functionality that was once used exclusively on PCs (what else did you have?) may now be more commonly accessed via mobile devices. You didn't have a PC in your pocket when you needed to look up directions on a map..