I believe Destiny 2 sold 38 millions copies before it went free to play.
Marathon will be multi-platform. Even though Bungie's reputation is waning, they have a pedigree with first person shooters.
Sony wants a GAAS hit. They will market the shit out of Marathon.
AC: Shadows reached 3 million players at full price and it was mired in controversy.
1-3 million players/copies sold at full price isn't generous. It's realistic. At $40, 5-8 million is probable. Helldivers 2 sold 12 million in it's first 3 months, half of that is manageable at $40 for Bungie. And if the game is cheaper.. between free to play and $20, I could see many people saying "Fuck it, I'll give it a shot."