Problem is that it can turn into a slippery slope. Where should MS draw the line if they start to allow Series X exclusive content? Can developers cut entire game modes from the S version if they just ask kindly enough? Or maybe ignore the S version completely? The risk is that developers are going to abuse this opportunity.
MS wants people to see the Series S as a viable purchase. Why should you buy it when you won’t be able to play the next big release in full?