I don’t dispute the Buddha did seat cross legged, all in all he tried himself the whole enlightenment-through-pain thing when hanging out with jain-like freaks!
I even bet he did as well squat for many days and months while with the niganthas, Mahavira is actually told to have achieved omniscience (kevala ñana) by squatting!
My point is that as per MN139 what makes the path rediscovered by the Buddha unique is indeed the aspect of how the whole natural causation of real awakening possible is not a given posture or a given external practice but in fact the individual discovery of an inner pleasure born of seclusion and contentment. The pain that leads to gain in this path of no conflict is an internal one, it relates to giving up habits, making life changing choices towards renunciation, etc - it is not about hurting your knees and joints!
Maybe that is not your case but every single minute I spent cross legged and just observing pain, elements, kalapas etc, was a minute wasted and away from the possibility of contentment, rapture, tranquility, happiness and stillness.
Hence I reiterate my question, is there any rule in the Vinaya, any quote in the Suttas (or even any summary in the Abhidhamma) which rules out a simple chair, or a multi step setting where one may sit and practice as something wrong or non conducive to awakening? I don’t think so…
I actually believe that the opposite may be very well true. There is an element of blind grasping to rituals in this wrong understanding of only one way of sitting being representation of what Buddhist meditation practice should be. It may be well be a key reason for so many giving up well before the very first steps having been taken.
I am really concerned that this fallacious and weird veneration of sitting on the floor may stop us from doing things within our reach for those in poor countries to having access to proper chairs and desks for studying and working!