The consensus seems to be that SO has peaked some time ago although I still get a lot of value out of it. Why then does nobody seems to think there's an upside to the acquisition? Maybe they can fix some of the problems.
What would an upside even look like? We expect SO to be free, which it is, always online, which it is, and have decent enough moderation to weed out the nonsense posts, which is generally the case. So the best thing that can come out of an acquisition like this is that nothing changes, so there just isn't really anything to get worked up about one way or another.
There don't appear to be downsides in this acquisition, but acquisitions usually destroy the thing that gets acquired (because they either get overhauled, regardless of whether it needed it, or they get folded into a different product, destroying its identity if the old product is even kept around at all), so the expectation for an announcement like this is that the acquired thing is now living on borrowed time.
We'll see what happens.