You know, the weird thing about Xbox's decline is that with most things in history, there is usually some debate to be had about where things went wrong. What source caused these issues. But with Xbox ... it's, really not even up for debate. It's Microsoft's focus on Kinect, or more like it's focus on just Kinect.
Frankly, I'm not going to diss Kinect or call Microsoft stupid for pursuing it or anything like that. The reality is that the Kinect was really too popular in the 360 era to make fun of, and I think it's silly how some gamers whine about it. I even think bundling the Xbox One with the Kinect was probably pretty reasonable at the time, though much like the pursuing the Wii brand further didn't work for Nintendo, it didn't work for Microsoft.
But focusing all their expansion development resources into Kinect? That's what made the Xbox brand fall. It's not even the Xbox One, because that was actually the fastest selling Xbox console for the first 3 years. It's not the Series S or X's tepid launch, because that was also the fastest selling Xbox for the first 3 years! It's the fact that Xbox didn't foster talent during the end of the 360 era, and instead focused entirely on Kinect, coasting their growth on Halo, Gears, Forza, Kinect and third parties. Coincidentally, if they had started fostering talent during the end of the 360 era, they would have probably released new games around the time Xbox One sales started to lag behind.
Now the conversation over what "fuck ups" Xbox made so that they can't beat their competitor is another topic, and really there's nothing Xbox could have done to win against Playstation.