You're forgetting one key detail; if you look at the, let's say 10
most played Xbox One games, only
one was available when Halo 5 came out, and that was GTA V, a previous-gen port. Roblox was a month later and Siege/Ark came 2 months later. It's much easier to get a high attach rate when there's nothing much else to play, hence why BotW actually had an attach rate of
over 100% at the Switch launch, as someone else said.
You're making an
enormous assumption on the number of people that actually want to play the game. Just because people have access to these platforms doesn't mean they want to play Halo. Unless you can find something that says otherwise, it's unlikely Halo 5 even sold
as much as Spider-Man, let alone more than. 5m in 3 months is great, but the rate at which that number grows reduces rapidly, especially considering the first 3 months of Halo 5 were the holiday season.
Infinite isn't suddenly going to appeal to a giant new userbase of people that are willing to drop $60 on it when they can get it on Game Pass.