We are just using Activision price tag as to how MS can invest on Xbox.
Taking this at face value for some discussion. I'm not sure this is even how MS are funding ABK e.g. they couldn't really just take ABK $70B and instead invest $70B cash anywhere near same timeframe nor benefits nor guarantee outcomes. The tax reduction/avoidance alone, which forms a large part of this acquisition, would not be applicable e.g. MS ain't spending $70B directly, they have huge profits and want to write down taxes via buying assets that will grow further again. I doubt the same would apply with investing over years or decades to build studios/hires/equipment and all that. Essentially depreciation over years of self grown vs instant write off in terms of same fiscal year benefits from buyouts.
I'm no accountant or auditor but I would heavily doubt the financial/tax merits of trying to compare a buyout vs organic growth.
*I agree they should invest in studios and titles but I disagree with your statement.
**It's not like they haven't purchased, started up, grown and expanded ID@Xbox, Rare, 343, Playground/Turn 10, Mojang, Ninja Theory, Kinect/HoloLens, Undead Labs, The Initiative, Compulsion, World's Edge, xCloud, Double Fine, Obsidian.
Given the investment in their current hardware, accessories, ecosystem, Game Pass, cross play, parity releases, streaming and ID@Xbox I'm very happy with their investments from a gaming point of view. Would I have liked some more AAA tent pole games over the last 5 years? Fuck yes. Would I have liked Halo to have been on song under 343 for the last 5-10 years? Fuck yes. I don't even have to go into the success and enjoyment from Game Pass, it's directly helped fund, develop, release and get more games of more genres in front of gamers.
I see them doing all things across the board. ZeniMax/Bethesda/Activision/Blizzard are huge gets with many facets and I see them not just set to continue what those purchased entities do best but also collaborating with all the other efforts of Xbox/MS e.g. Game Pass and more; studios will benefit from the talent, assets, engines, support and more. Given the factory type output of say COD it looks like Xbox have more options and diversity of titles out of Game Pass e.g. it's looking pretty great in terms of what games devs get to make and what games we get to play.
Now we can look at the differences of invest and grow your own vs buying known quantities and brands. Would you invest $70B in the unknown and have to foster everything from the ground up while taking a decade or more to achieve what can be done in say 2-3 years with buyouts and known IPs/talent? It's a pretty easy answer.
For something a little more direct, I'll take buying ZeniMax over creating/running 343 any day of the week.