But you're exaggerating the period in which they will even support cross-gen. And again, Sony will have at least a few cross-gen games as well. 3rd-parties will definitely have cross-gen titles. Cross-gen games are usually done by contracted 3rd-party studios. Engines are massively scalable in components going from graphics to AI to physics to logic. Etc. etc.
I explained every factor into this and why your notion doesn't hold up, including the actual time range for 1st party mandate on cross-gen support, how titles between next-gen and cross-gen will be developed (top-down, not down-up), and the fact that there really hasn't been full clarification from MS on how they will implement the cross-gen support. It could be through streaming via Xcloud for all we know. That would certainly be an option for supporting older systems down the line, without incurring any design limitations on 1st-party titles targeting XSX natively.