JB is unquestionably the right guy, he's just not the finished article yet.
He's big and hard, and that is a major advantage for a 10 (especially a small one like McKenzie) as it gives you a bail out option that makes sure you can retain shit ball. Nonu provided that for years.
He knows where the space is - admittedly he's not that good at getting the ball there yet, but he does see it - and that is the bit which is very hard to teach. Accurate and fast passing is much more learnable, and he will get there - he spent a long time playing at fullback where you don't use those skills regularly.
He can kick.
He can tackle, has size to stop momentum and knock forwards over, and doesn't often fuck up defensive reads / come out of the line. Not perfect by any means, but he'll get there.
He's very committed. And generally I think he stand up in the big games and his temperament has improved miles.
I wasn't a JB fan early on, didn't think he should have been picked at fullback and definitely not wing initially. Now he's the best we've got, and has the potential to be very good.