No game is nearly as important right now as Halo Infinite. They've already set it as the centrepice of their new system/launch, and they're marketing it as a true return to form as well as their main showcase for next-gen capabilities. While there's a lot of fans still jaded over the post-Bungie era, it'd be naive to then believe Halo doesn't still have huge presence and relevance in the medium, especially when it comes to the Xbox brand.
To piggyback on one of Rol's points, the most important market for the twins are the gamers who'll buy the best-selling mult-plat games i.e. CoD, GTAV, FIFA and so forth. One huge advantage Halo has as a showcase exclusive, is it appeals directly to the exact same crowd who buy those games. A bit different from the crowd who may have more interest in streamlined/story-driven games, artistic games, and obviously a lot different from the main crowd who like Nintendo games.
Even being optimistic, the only exclusive PS5 might have near that calibre in it's first year is Gran Turismo 7-- a sequel for a series that has been in serious decline, and will probably be outmatched by the new Forza.
There's a few important prerequisites for the system, like price, functionality, broad AAA support and not having another marketing disaster like last gen. But after all that, Halo Infinite is the most important thing by far; followed by other exclusive offerings during the first year; cross-gen support; services; and showing what all their new studios will be offering going forward, while proving they'll be committed to those projects and won't just can them and/or the studios. For that last point, it'd go a long way if they could revive Scalebound's development, and have it as a cross-gen release in the first year of the new system, but that's just a personal hope.
