Usually I tend to go with the coach and the club over players especially overseas players. However in this case no, if anyone didn't didn't 'have the right attitude to be even an average coach' it's Agar, his attitude far exceeded his abilities as had been proved before and since.

He was at least to blame for all three, he was a poor coach who hit lucky for one season.

John although I don't like to admit it, did have a bad attitude though. Seemed to hit some form after Wakefield and picked up some first grade NRL experience along with an international call up but it didn't last long and he ended up calling it a day to pursue YKTR (his clothing brand that he runs with James Segeyaro and Corey Norman).

I'm no big Agar fan though I don't treat him like the devil some do but development is more about the individual.
By all means say another coach might have done better but he didn't exactly do them harm.
They all had chances in the first team.
In John's case he didn't take it seriously.

I'll accept John to a degree but Cowling and Wallshaw never got a serious chance. Wallshaw in particualr played well when used as a centre to which he was rewarded by being shunted into a pack where he wasn't even needed. Yet as someone else noted he had an obsession with Trout as player nobody else had rated before or since. Sorry but I think he did a great deal of damage.