I do not think this is fair or true to a large degree. I think Rondo's shot improved marginally over the years but a lot of his progress was halted by playing with three hall of famers who could carry a team. It was their team all the way and Rondo was the fourth option. He also benefited from this making an all-star game.
I thought he could compensate for his lack of shooting until the injury with his speed. He could drive by folks and penetrate but once his injury happened he was never the same player. Typical Ainge guy who has some talent but if flawed in some way. Ainge loves to gamble that their strengths will overcome their flaws. For a while, with Rondo he was right.
I thought the season in which he got injured he was shooting the mid-range jumper really well, so he did develop on that regard.
His biggest problem has always been on the defensive end (which is maddening since he's a very capable defender), the lack of penetration (particularly during the season), and his inability to "run" and walking the ball up the floor. Well not inability, but he found it difficult to get himself to do it.
That's always what doomed Rondo more than anything, not the lack of shooting.
There's always Playoff Rondo to look out for, which makes the analysis of him as a player a bit difficult, though his inconsistencies also remained, but he was a top performer when he put his mind into it.