It's not the lack of monitor, it's the fact that the monitor and keyboard are inseparable. If one is well-placed, the other isn't. You can fix this with either an external monitor or with a laptop stand and external keyboard.
maybe i'm addicted to something I shouldn't be, but I much much much prefer laptop/keyboard/screen setup to external monitor/keyboard setup.
I use an external monitor from time to time, but I'm too mobile to have that be a "default". With the laptop setup, I have consistent experience wherever I'm at.
That said, I'm typically reclining (feet up on desk, leaning back, sitting in easy chair, etc) for most of my laptop time, and perhaps the experience is different from just have it on a desk most of the time?
Posture (neck angle) and arm/wrist placement are really hard to get right in the situations you describe.
I saw a physiotherapist at one point and the best advice I got was to get someone to take a photo of you in your "usual" position. You can analyze that yourself or bring it to your family doctor at your next appointment if you are curious.
Laptop use and poor posture go hand in hand. An external keyboard can also work as it allows you to move the laptop display to a better position, but if you're going to do that you might as well go all the way and get a decent monitor too.
This is news to me, can you explain a little more why using a monitor is better than working primarily on a laptop?