Joy-Con are such a cornerstone of the Switch's identity, such a key feature, an icon all to themselves, the LITERAL, ACTUAL LOGO.
They aren't THE POSTER CHILD.
They are THE POSTER.
Their use as individual controllers is not minor nor niche, Mario Kart, Mario Party, ARMS, 1-2 Switch, Super Smash Bros. Ultimate and many more all either support or require single Joy-Con play.
As I've said before, I think this site and indeed this thread can be a little insular. I would think we are mostly closer to the "hardcore" side of gamers. We have a Pro Controller or equivalent, we leave the Joy-Con attached permanently and barely even think of them.
I want to be extremely clear, this is not the mass market. This is not how the mass market treats Joy-Con. In the eyes of parents this is "the system is a handheld and comes with two controllers, wow!", in the eyes of broke college students it's "I don't need to spend an extra sixty bucks to play with my roomie? Sweet!", in the hands of casual gamers who find themselves more immersed by accurate motion controls than button inputs, they are what enable their sessions of Ring Fit Adventure and Fitness Boxing. Families buy one extra Joy-Con pair and they're sorted for every family game night of Mario Party, or 51 Worldwide Games, or Monopoly.
Joy-Con are a masterful gambit not in design, though their size for their features is impressive, but in sheer market penetration, sheer people pleaser factor. For hardcore gamers they are a full set of controls attached to what was for a time the world's most powerful handheld, for the train commuter they are a full set of controls they don't have to pack for the train ride, throw it in the bag because they're attached to the system and fit in the carrying case (this one is me). They satisfy nearly every segment of the market with an unmatched elegance, and no matter how maligned the hardware, not for one second can I justify even entertaining the idea of a Nintendo hybrid without Joy-Con.
Nintendo's Switch era has shown they are masterful at marketing, for better or worse, they are good at it. I am certain they know all this already. Joy-Con were the result of developer input internally and became a product that defined an entire formfactor. They are a success story with obvious flaws, and if the next generation truly is bigger, there's easy wins to be found in improving them.
Permanently attached controllers are for handhelds. Nintendo Switch and NG Switch are not just handhelds.