Do you understand anything about basketball?
You look at Danny when he was hitting the 3 well, he was basically a spot up shooter. He couldn't even dribble to the rim without turning it over most of the time. Now he's trying to take guys off the dribble, so that hesitation, should I shoot the 3 or drive to the rim... that changes something about his shot. Some shooters, great shooters, you cannot change anything about their shots without negatively impacting their stats.
Plus the Spurs don't move the ball as well as they used to, so Danny doesn't get wide open looks and when he does, he spends too much time sizing up the shot. He used to just let it fly. Tim had the same problem - he'd miss shots he had all day to take but hit shots he had to release in a hurry. Too cerebral to be a great shooter, since shooting is a muscle memory and almost involuntary response to having the ball in certain situations. It's like catching a ball without watching it come to you. You just know where you hands are and the arc of the ball and the speed, and you don't need to do the math. Danny is doing the math.
So yes, playing constant defense causes tired legs, causes a lot of things. That's why people like Steph don't play much defense. It's why guys like Lebron don't do well in the 4th quarter most of the time. NBA2K players think it's choking, but something about the player has changed just enough to effect the percentages. These guys are at their pinnacles, basically the best they can ever be, so it's all downhill from there.