I don't think "but we've done worse things" is a good argument to allow bad things.

The reason ActiBliz + MS deal it is getting scrutinized more is because Lina Khan became head of the FTC, and she looks at mergers and acquisitions with the same dislike that everyone had towards Standard Oil monopoly and AT&T monopoly back in the day.

The Obama Administration had a very lax antitrust policy. For example, they approved the Ticketmaster + Live Nation merger despite it clearly being a vertical merger that gave a single company control of the majority of both the venues concerts were held at and the tickets being sold for those concerts, ultimately resulting in the Taylor Swift fiasco that was in the news a couple months ago. Monopolies like Ticketmaster are complacent because there is no one to compete against and therefore no reason to make things better for the consumer. Things have changed because the head of the FTC (and many other government agencies) changes when a new president gets elected.

People like to only focus on how prices change as a result of mergers, but until the 1980s everyone including judges also considered the political and social cost of mergers, in addition to the monetary cost to consumers. Maybe if we continued to do that and didn't largely stop in the 1980s we would not have too-big-to-fail banks or a mobile app store duopoly.

What annoys me in the UK is how much monopolisation has messed up out public services like water, public transport and energy, yet we do very little to reign any of that in. However when something non-essential like gaming comes up, we move to take action.

You had it right in the first bit. MS is legendary for it's greed and general evil shitbaggery. Gaming isn't somehow different from all of the other industries that have fallen to monopolies. Consumers are always worse off after diversity has been crushed in a sector.