At some point, the current flight simulator products will fall woefully behind. Some change to GPU technology will either make them non-functional, or they'll just be so far behind in technology that people won't want to use them. Additionally, at some point Microsoft will decide that MSFS is no longer worth the cost of support and producing disks, and they'll drop the product all together. At that point, PMDG will have the choice of either creating an X-Plane product, or transitioning to the Cascade Game Foundry product (I'm assuming here that this "tipping point" comes several years down the road and the CGF product is released)