The spot was right. As right as any human being can get it.
Look, the receiver's body was sliding forward even as he fell. He was down by contact because of his right thigh brushing the ground. Even as he was moving forward. When you call him down, when that thigh pad touches grass, that's very much a judgment call. And the person who can see that thigh can NOT see the ball, on the opposite side of his body. And vice-versa.
These refs aren't omnipotent. They do like you and I do, they just do the best they can. And the spot looked about as accurate as a human being can make it without x-ray vision.
So how about we stop crying about what we WANTED and accept what actually happened on this play? We didn't successfully convert to a first down. Period.