Do you have a crystal ball in front of you or something? Though the chances of him going back to his 03-04 form are slim he will probably do better than he did last season.
Awareness of the inevitable passage of time and declination of skills? Yep.
Peja's threes are no longer money in the bank. I used to KNOW as soon as he let go that it was 3 points on the board. Slowly but surely that changed. Towards the end of his time in Sacramento, I would hold my breath. There was nothing automatic about his shot any longer.
In Indiana, he didn't do much better.
You asked why people are questioning Peja's ability to hit threes. I think my answer and some of the others, above, are pretty clear.
Peja peaked. I don't think he's hungry any longer. He never did live and breathe basketball. His peak came when he won the title with Vlade and the three-point shootout. He's been in a mild decline since then. And we all know he isn't working on shooting over the summer. He made it clear the off-season was going to be for his family, which is fine, but isn't going to help his game.
Peja is Peja. If you expect too much of him, he'll break your heart.