Not wanting to turn your head 90 degrees to see your 13th monitor is not a "mobility issue".
At least you have the option to put monitors above and below as well.
And completely swap configurations for different use cases e.g. coding versus gaming.
I agree with the parent that any setup that requires me to turn my head to see all of my screen space is a downgrade, not an upgrade. Even a monitor that's too big (above 30 inches or so at normal desk viewing distance) is bad.
If you like it, go for it, but don't act like it's the only or even most common way to work, even for developers.
And there has been quite a bit of research [1] on them with 98% of users preferring dual monitors.
[1] https://www.ie-uk.com/blog/how-multiple-monitors-affects-pro...
I've never seen anyone using "a laptop and an external monitor" who actually uses the laptop screen. (Where by "use" I mean "looks at it." They might have it on, but it's usually just idle at the desktop.)
Personally, I plug my laptop into a monitor and then put it, closed, onto a little stand for ventilation. (One of these things: https://www.apple.com/ca/shop/product/HP9X2ZM/A/twelve-south...).
I use it as a screen for my slack/discord/email and have my two main screens above it. It's true I use my two main screens more, but if I didn't have my laptop I'd want a small third screen to replace it.
But if you have your laptop sitting directly on the desk — presumably because you use its keyboard to type? — then any time you look at its screen, you're straining your neck. There's a reason monitors are on stands that hold them up 8+ inches above the desk — it's so it doesn't hurt to stare at them all day.