That is $2.8m per year to pay for the development of that all free multiplayer DLC in that hypothetical scenario. But for Halo it would be alot more. 343 Industries has nearly 600 devs, I would assume there will be at least 150 people on Halo Infinite's post-release multiplayer support team, and MS has already said that they want to support Infinite the entire generation with both singleplayer and multiplayer DLC. While the singleplayer expansions will presumably cost money, all of the maps, modes, and weapons that will be released for Halo Infinite's multiplayer will be free. So 150 people making an average of probably $80,000 per year (343 Industries is in a high cost of living area so I assume their average pay is higher than most other AAA game studios), with probably 6 years of support. That is $12m per year x 6 years = $72m. You can't just expect MS to be willing to spend $72m on 6 years of free DLC support for Halo Infinite's multiplayer, it's unreasonable, even if the game will make over $1b in profit from sales of the game. They already have a rumored budget of $200m+ for the past 5 years they've spent working on the game with 500+ people.
This is the real world we live in, gaming is a business for AAA game publishers like Microsoft; they're not in the business to earn the goodwill of gamers, they are in the business to earn as much money as possible so that Satya Nadella and the other MS executives can be filthy rich, live in mansions, bathe in champagne, and fly in private jets, the American dream baby xD
The good news is, Halo Infinite microtransactions are confirmed to be cosmetic only, non randomized, and earnable in-game. I can't really complain about that, I would only complain if they were randomized, it sucks to want a certain cosmetic and have to rely on gambling to get that cosmetic.
What? I can't expect a company to spend 7.2% of it's total sales profits on free DLC and updates? 72 million is 7.2% of a billion. That's like a company making $10 in profits from a sale of an item, and then giving their customer a 72 cent plastic gift.