Halo Infinite is going to do ridiculous numbers, just watch. By then, they'll have a working cloud system to be played on mobiles and such. It will launch simultanenously on PCs through Play Anywhere. All Game Pass subscribers will have access to it, and if you aren't subbed yet, there's always deals to jump in ridiculously cheap. They could be looking at dozens of millions of players in the first few months alone if they play their cards right (note: players, not copies sold). And well, the game doesn't even need to be a timeless masterpiece with 97 on Metacritic, a solid 85 or so will do: it just has to feel fresh, provide a stunning open world, and well, the rest will come on its own since the core shooting mechanism has never been an issue with Halo, really.
I know this is easier said than done but they have the capabilities of providing all this, and from what we know about Infinite this seems to be a likely scenario. Halo may not be as big as it was with Halo 3, but that E3 reveal generated tons of hype, people were digging what they were going for big time. They have to play their cards right and Halo Infinite will "easily" be the biggest Xbox One exclusive so far, definitely bigger than Halo 5, the Forza games, Gears, etc..