Although I haven't tried a standing desk, I know enough about such ideas to know this would be my experience:
Funny...but the picture of him sitting slightly back with legs elevated is the way I've been sitting when I'm at a desk for most of my life. And that's despite regularly being told it was "healthier" to "sit up straight and keep both feet planted squarely on the floor." Slouching back
(as I call it) seemed to be my preferred posture for anything requiring long periods of concentration or brainwork. The body just seems to disappear and the mind go into "the zone" whenever I'm seated like that. (Although after all these years of repetition, I may have just conditioned
myself to focus whenever I'm in that position.) I've discovered I can remain standing to draw, build, or do something active
. But I can't remain constantly standing and truly and deeply concentrate on something. I will
get up and pace around - or stare out a window every so often while working on a 'mind project.' But my best 'thinking position' is definitely seated and fairly stationary. And I don't think I'm all that unique in that regard.
Most of the people who I've heard advocate for standing desks are noticeably athletic younger people with no disabilities or old injuries to deal with. They're like those trainers you occasionally run into at fitness centers who've been decidedly fit and healthy their entire lives and simply can't comprehend anyone being any different.
You know the type.