Before getting hurt last year, he did look great offensively. It wasn't just about shooting. He was a scorer, who got his own shot. He just plays for a jumpshot. Doesn't even move off the ball to get it.
That's my big problem with your argument. It was a 13-game stretch. That's a tiny sample size compared to looking at the 2,3 or hell even 4 years. You can't just pick out the absolute best stretch of his career.
Players' production generally doesn't improve much past their 4th year or so. There's always some exceptions but look at the majority of non-high schoolers and it's a strong trend.
Bargnani is what he is. 27 years old and in his 7th season in the NBA. That is an established veteran player. Unless you are banking on him being an exception to the rule, there most likely is no improvement left there.