Anyway, I do think the Live/Legacy split could be the final nail in the coffin, if stuff like account copying hadn't already been enough, for any hopes being held out for a native 9th gen Odyssey client. 3.8 on consoles is running in their backcompat mode. Introducing a new version would mean having two SKUs on each console's store in order to keep supporting Legacy, this means no Smart Delivery on XBox, and thus essentially marketing one game confusingly as a sequel of sorts, ironically probably how they ought to have gone about Odyssey's mechanics in hindsight, building a game for them from the ground up instead of bolting them on through dodgy knots in spaghetti code.
(I guess TECHNICALLY they could build a launcher similar to how the Rare Replay collection handles its bundled 360 titles, or the Halo MCC's menu option to play Halo 5, which would contain a Legacy option in the menu that would launch the 3.8 client in backcompat mode and suspend the current gen launcher)