In terms of technical and game development side they better focusing on one console and optimize it for one console (Scarlet) so it will be a technical demo on how powerful Scarlet is and can be a selling point. In terms of economy and business side they have to release to both generation to achieve targeted sales and revenue.
Microsoft has recently changing their focus from selling Xbox console device focus to selling more games and service to other device , thus they has changed from devout to being agnostic. On next gen. Microsoft will focusing on Xbox as family not just a single platform.
Xbox has changed from a console to a platform and brand thus they want to avoid being "exclusive " to single device and trying to reach more market using other non traditional method A.K.A Console PC and streaming. They are slowly but sure trying to migrate to streaming and PC.
The reason why Xbox Scarlet were having three SKUs planned , one is Anaconda, the second one is Lockherath and the third rumored ones is streaming device , it's just shows Microsoft is trying to make Xbox to be more like PC and Steam machines and reaching consumer outside of dedicated hardcore gamer.
So if we speak in 2013 with console focus mentality, Halo Infinite probably will be ended up as Scarlet exclusive but now we are talking with Xbox division sirca Phil Spencer where revenue and sales of games is more important than selling one single console device. So they will not care if Halo will be holding back by Xbox One graphic capability.
