I think that's kind of a gatekeeping statement: "X is what D&D is, Y is not!" is rarely categorically true.
You're absolutely right there, and you make some great points. The fact is, I know from my own experience that there are people who love character backgrounds, who love keeping track of downtime, who love kingdom building, and who love mass combat. At various times, I've certainly fitted into those four categories. My players and I enthusiastically adopted the Stronghold Builder's Guide back in 3E, and we lamented the lack of rules for actual kingdom building. And we were all enthusiastic about playing Kingmaker.
However, running Kingmaker showed me what actually having these rules could do - and it wasn't all positive. So, I'm now interested in exploring the options. I don't have a settled position; my first post was just that, a first post, and I'm fascinated in the feedback I get. Yes, my posts will be clumsy as a result, but it's a topic I consider worth exploring.
Cheers!