Do you really think Microsoft is going to raise the production of Series' units or stock them for GTA VI? By the time it drops it will be 500k per quarter. How much stock should it have for a meaningful boost in HW sales? 2 million? 5 million? I honestly don't see Microsoft stocking a year's (or more) worth of Series' units for the launch and even less at the end of the generation risking entire warehouses with unsellable merchandise.
GTA VI's impact on Series hardware will be minimal cuz the production will be the bare minimum by then if it's in production at all. And it will be worse the more it's delayed.