So many of my breakthroughs happen on walks. Being forced to reconstruct the situation in my head makes me think about the nitty-gritty details in a much more systematic manner.
Oh, but chairs for me are not a problem. I have been a very happy standing-worker for 13 years now. It started with hip and lower back pain in my early 30s, which seemed f'd up, and then some articles by a Mayo Clinic doctor who worked standing and sometimes on a treadmill. Turns out, it was good advice. Now I stand all day - sometimes 18 hours, and don't even notice. (To be clear, you always move a bit, shift your weight, etc.; it's not being a statue.)
I think it's all a balance of sitting and standing, and not taking one to the extreme. I say this as I just threw my back out a few weeks ago, because I was sitting all day :( So yeah, I'm not a model person to listen to either!
Slightly more seriously, there is no one answer. But in terms of total human history, sitting isn't something we spent a lot of time doing. Sleeping, lying flat, we obviously spend about 1/4 to 1/3 of our time doing. Standing or walking or running surely must be a big portion of the rest. Sitting would only be reserved for when you have the time to "do nothing". That's kind of a new concept. Thusly, I'm not afraid.
For a 50 year old nerd who hasn't exercised in a year, I'm still well above the health and capabilities of a lot of same age people. And when I'm in a good environment with outdoor play areas (mountain biking) or indoor gyms, I'm fit. Aesthetically, losing hair and gaining less elastic skin is much more a concern to me than some tiny purple lines in my ankles or knees :/
Going outside, working just off the laptop, is a definite shift. It usually means less coding, but sometimes it pushes me to be productive in areas I normally neglect (documenting, planning, organizing).
To do real development, I am very dependent upon a big monitor. If I travel, I take one or rent one.
Honestly though, sometimes I just go for a walk. And often during that walk, with no screens, my brain magically sorts through some problems and gives me new insights.
If you work from home though, most people agree on dedicating some space and properly equipping it. Working always only on a laptop in a floater kind of way is something that I suspect most people cannot do with any real level of productivity.