Wasn't Tony Parker a pretty poor shooter when he first came into the league? He's far from lights-out, but he's decent enough to open up his penetration.
Maybe he was a little younger than Rondo when he developed it.
I don't think Rondo will ever be good. I'd be a little surprised if he even got decent. If it was strictly between the ears, I think he can handle it. If it was just his form, that's also correctable. But with him it is both, and that's very difficult to overcome.
I heard your muscle memory ability peaks in your 20's, so the older he gets the less and less likely he is to develop any consistency. He's got shooting coaches, practices, short offseasons, long offseasons, you name it, and it isn't making a noticeable difference.