I don't know why either. I personally like the Klingon War (as a matter of fanon) as well as seeing more of Sarek. The rest of your points are all fair and well and good. As I said, if I were making a series set ten years prior it would look a lot different.I think with me, the biggest issue I have is: just why someone felt the need to make a new Star Trek series and have it be set ten years before TOS, while at the same time not really resembling TOS all that much. Seeing what I've seen so far, there really isn't much story-wise that needed to happen in the 2250's. Let's see:
1. Klingon war. Not sure why this subject couldn't have been done at any other point in ST history, especially since there's no indication in TOS that there was ever a war ten years prior.
2. Sarek being a foster parent to a human girl. Well, Sarek was still alive right up to the middle of TNG, so he could have fostered her any time between TOS and TNG, and probably would have been better off after TUC, instead of Spock having this sister throughout TOS that we never heard of.
3. A new super-secret star drive. Ok, kinda odd that no one ever heard of the spore drive before, which probably wouldn't have been as problematic if it was invented, say, post-Nemesis.
4. The Mirror Universe. Heck, I'm not even sure why it was necessary to even have this in DSC, other than to have duplicates of some dead crewmembers cross over to our side. Which again, could have been done at any point in ST's history.
Did I miss anything?
No, I don't know why they did it the way they did it. We probably never will know. And, frankly, I think Star Trek is struggling to keep up, so I am uncertain as to keeping it around. That said, the fact that I wouldn't do it is why I want to see more of DSC.