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.