I don't think they are spending $70bn just to make some side money from third party releases.
This acquisition isn't meant to make money like that. It's supposed to buy Microsoft more control over the industry.
Activision Blizzard has a yearly profit of between 500 and 700 million.
Even if they keep that up or extend that (which is pretty much impossible when you decide to limit your audience) it would take them like half a century to make back their 70 billion.
So this is not an acquisition meant to increase MS profits, but rather one meant to extend monopolistic control over this industry.
With Metaverses on the horizon, I think Microsoft realizes that Gaming is the key to the Metaverse idea.
By controlling the gaming industry you control the tool that lets you comprehensively monetize people's lives by gamifying all aspects of it: Work, social, recreational.
There are only two relevant metrics from a business perspective: Engagement and monetization.
Anyone can monetize, but Zuckerbergs ridiculed Metaverse Meeting nonsense or the internet's reaction to stuff like Metaverse marriages clearly show that in order to engage people constantly you need to apply all the expertise of the gaming industry.
Metaverses will need to compete with the best games out there on a visual and mechanical level at least. They will essentially be games.
Neither Google nor Facebook nor Apple has that kind of expertise, so I think Microsoft is trying to make sure that they have their thumb on it.