Though if we go with strictly the install base argument, I could see the EU approving the deal, but the CMA ultimately saying no due to requiring remedies that MS are unwilling to adhere to.
I don't see why that would work as a convincing argument for the EC, either. The install base difference can be rationalized as Sony offering more of what the market wanted, and doing so without consolidating a massive publisher. If MS's history of neglect towards the European market during the XBO were put on full display, everyone would easily see that their market position is 100% their own doing, and buying up massive publishers isn't going to suddenly resolve that. It doesn't even guarantee that Microsoft gets their ass in gear and makes smart use of their acquired assets.
If I were someone present contesting the deal, I'd bring up Microsoft's history of failed utilization of acquired assets dating back to the original Xbox, or their lack of helping acquired teams grow and achieve more in terms of industry-leading content, and in many cases even in terms of generating more revenue. I'd also point out their history of IP mismanagement, such as the fact their once-marquee FPS IP Halo now being a shell of its former self, is obviously a motivation in them wanting to buy ownership of the #1 FPS on the market that their own studio was unable to win in competition with due to Microsoft's incompetence.
Like, actually get to the meat & potatoes of why Microsoft want ABK and then reinforce the fact almost every problem they are trying to solve through the acquisition, is a problem they created for themselves and in which the acquisition doesn't inherently solve, nor do Microsoft as a platform holder have a history of leveraging acquired assets to fix foundational problems with their gaming division.
I mean, MS have basically already said multiple times
"We kinda suck at this gaming thing, let us buy ABK. Let us compete!". But they've only been telling half the story. If I were Sony, I'd tell the parts Microsoft doesn't want to tell.
cbots on era are pretty convinced the deal will pass
They never lost faith. Even I have generally been of the opinion the deal will pass, but not with the weaksauce behavioral remedies Microsoft have presented so far. There are going to be concessions involving a structural remedy or two and that's the one Microsoft seems most afraid of having happen.
Which means it would be the best solution in order approve the deal. Granted, I think it could be a 'lighter' structural remedy given the behavioral ones Microsoft have already proposed, but I still think it should involve divestiture. I just don't see why they can't retain partial ownership of the divestiture (though, they probably should not be allowed to buy further ownership via shares in the divested entity, either. At least not unless other shareholders with stock want to sell to them...I'm not really sure how that part would be handled from an enforcement POV though).
The warriors in places like that ResetERA thread want the deal to go through with no concessions or the most babyish of behavioral remedies for Microsoft. They're either unaware of the floodgates that'd open for rapid mass consolidation in the gaming industry, or they simply don't care. And if they don't care, they were never really fans of the hobby (or its history) in the first place, IMHO.