Not directly, but part of their popularity does seem to be a response to studies showing that exercise doesn't offset sitting. There were a few pretty convincing results in a row concluding that extended sitting is harmful in itself, regardless of exercise frequency. Given that many other studies show extended standing is also bad, I think the question worth investigating here is whether position changes offset that or whether interspersing actual movement is required. (More speculatively, a major leg-bouncing habit or an 'active' base for sitting or standing might help.)
> Sit/stand desks have an added bonus of letting you adjust the height of the desk when sat down, which makes a big difference.
This is reason enough for me to value a sit/stand desk - even if I never stand up, the ergonomics of controlling desk height are a huge benefit. Guides to adjusting chair height for that are fine, but I want to adjust my chair relative to the floor so that my feet lie flat, then my desk to my chair. (And then, ideally, my monitor and keyboard separately also).
I agree that bad chairs are almost certainly a big factor, in the ways you describe. Circulation issues and back pain are two of the biggest issues with sitting, and they match up to pressure from the chair bottom and poor lumbar support. In addition to the padding issue you mentioned, chairs with back support are usually worse for me, because the back support is often fixed in place and only correct for people of one specific height. Either getting a high quality chair or being able to change positions often seem like the most obvious improvements.