I worked in tech consulting, and my colleague actually had a very thin ~23" (IIRC) portable LCD screen he somehow carried with him on planes. It had a foldable kickstand so he could set up a dual-screen on any hotseat desk in seconds. It was brilliant, and I briefly considered the same, but my backpack was already heavy enough. I've heard of folks doing this with the MBP + a iPad Pro, but idk how well that works.
I keep looking at those, though in the 12-14" size. I'm interested and can see use-cases.
But - you can put those to the side of your regular monitor.
It's replacing the keyboard ("underneath" / looking down from my primary screen) completely with a full-size second screen, that I don't see myself doing just yet.
Itβs startlingly good, even wirelessly. I spend a lot of time working between different offices, and being able to set the iPad up as a secondary display is very handy.
The iPad display also gets its own touchbar instance, and it makes way more sense as a thin touch control strip at the bottom of the screen than it does as a function key replacement.