I think you're over simplifying it - some NRL clubs have their Head Coach as a leader of the organisation, managing all team affairs and the strategy, whilst empowering their assistants to do the day to day coaching and implementation of the plans (Brisbane and Melbourne), and this absolutely does prepare the assistants for bigger roles - but that approach is not taken en mass.
Who are the assistants being groomed at the Eels, Warriors, Titans, Tigers, Raiders, Knights, Manly, Dragons etc? Even at the Sharks where Steve Price (ex Dragons head coach) is assistant, Flanno is still very much a hands on head coach, and they're certainly not 'grooming' Price.

Just because Bennet and Bellamy produce lots of head coaches from their set up, it doesn't mean all NRL clubs are doing this purposely - they're looking after their own interests first!

Clearly though, the NRL clubs have a bigger back room staff and use their assistants better then we generally do over here - and certainly better than leeds do.

At Manly was Toovey the assistant to Haslam?

Same one who now manages Bradford? Isn't that that the kind of level an assistant like Eaton might eventually aspire to? Or have I missed your point?

Once upon a time yes. Though again, I'd suggest that was more circumstance than a deliberate ploy to groom an assistant - Toovey had played there his whole career and moved into coaching. I think the fact Manly haven't 'groomed' a replacement since Hasler left (who were the assistants under Toovey and now Barrett?) suggests it's not a club ethos, and Hasler has been very much the top dog at the Bulldogs without grooming an assistant (the club actually forced him to change his backroom staff as they didn't like how much power he had), so that suggests it's not a Hasler thing either.

Most top coaches will have been an assistant at some point - most clubs are not running their organisation with the assistants actually being the main day to day coaches.

However, all that said, good assistant coaches who play a big part in the teams development are a vital part of the organisation in my eyes.

My take on McD is that his assistants have actually been Sinfield and Peacock, both pretty much being player/coaches. I get the impression McD is an excellent 'leader' who empowers the team to make decisions and forge their own way - a proven leadership technique. However, that technique relies on the team having the required competence - when that competence is not there, more direct management and direction is required, and I'm not sure McD has that in his locker.

Sports teams are cyclical, sometimes you need a leader McD or Bluey style, sometimes you'll need an authoritarian dictator Wane style or a 'detail' man like Tony Smith, sometimes you'll need someone in the middle of these techniques. It all depends on the playing staff at that time, and you'll usually need to cycle between the different styles to keep things fresh - now is time for more direction and hands on management of the game plan. Personally, I'd like to see McD stay involved in a leadership capacity, overseeing all rugby management (all teams, the community game etc), but put proven tacticians in the head coaching positions of each team.

The same one that many on here are suggesting would be a good replacement for McDermott - nowt as queer as folk