This is a cool idea, but simply will not happen because it's too complicated for Nintendo's audience. Heck, it's too complicated for most console players.
It's a nice idea for Steam Deck, though.

I do wish Sony would offer gpu upgrades. That would be cool.

Only possible in utopian future where everyone is adequately versed in the tech fields

Sorry if I didn't make that clear, but yeah by modular I of course meant something you simply snap/slide into and out of place. And of course you would only be switching the parts out once every few years when you update a piece, so build quality wouldn't be a problem as you'd literally only be using each connection (connecting or disconnecting) at most a handful of times. The only connection being used often would be the joycons....but that's already the case today with the Switch, the other connections on this hypothetical system would be getting used like less than a hundredth of that amount of time.

So there ya go, your reasons for it being very impractical are already solved by the initial idea. :)

Doesn't require any more being versed in tech fields that buying a new system does. People didn't need to be versed in tech fields to know the PS4 Pro was a 4k updated version of the PS4, or the Switch OLED was an premium version of the Switch with a bigger and better screen. When Apple updates the iPhone with a better screen you don't have everyone confused by what it means for the screen to be better. These are very simple concepts that everyone understands. No specs knowledge would need be involved in order to get an update, just like any other video game system or upgraded model.

It'd be as simple as Nintendo announcing a new better screen that is an update to the 3 year old screen. Or Nintendo announcing the chipset 4 upgrade that is an update to the last chipset (chipset 3) from a few years ago. Or Nintendo announcing a new battery part that has say 40% more capacity. None of that requires any technical knowledge from the consumer.

The only difficult part would be getting people onboard with the concept of there no longer being completely new hardware distinctions between generations and instead each game would just be playable by, for example, chipset 3 and up and won't have good performance on the older chipsets. It'd basically be a super simplified consumer friendly way to do PC-style system upgrades and do game minimum specs as to allow consumers to need zero technical knowledge but also allow Nintendo to do away with the generational reset that happens with every new system.

Not that I think they are going to do this haha. Just though it was an interesting idea that if they marketed it successfully it would greatly dampen for the future the problem they are likely currently worried about which is trying to bring the massive Switch userbase over to their next generation. Like, if they had already done that with the Switch, assuming sales were the exact same as they have been, then there would be no issue in trying to market next gen to the (eventually) ~150m Switch owners, it'd just be hey we've got the next chipset leap coming out, get it for $150 (or whatever) and slide it into the Switch you already own to replace the old chipset and play all the same games plus the next few years of "next gen" games.

Okay... but *how* do you make a screen and chipset easily modular without sacrificing a robust build that can survive being used on the go?
It's easy to say an idea is not impractical if you handwave away the difficult part of building it.