I never said that FO's can't be good teachers. Read my posts again. Nine out of ten times there will be no problem what so ever. But there will be some areas in which the training FO simply has no experience. If you are willing to overlook that deficiency in a trainer, fine, bring in the training FO TRE!
But who will conduct base training
Who will do the command assessments?
Who will do command coaching?
Who will do teach the command course?
Who will line train a new commander?
These questions show the obvious inherent problem with a training FO. But it goes deeper than that. For example, when a captain is undergoing recency training, the trainer will be in command even when he flies from the right hand seat. Would this be possible with a training FO? I have no principal objection against a training FO, but the practice can be problematic.
And regarding the man-boy relationship you talk about, maybe it is so in your base. We have no cadets in my base and believe me, there is no man-boy relationship over here. FO's here are vastly experienced, have flown all around the world, for many great and not so great airlines, or have built up experience in other bases. I deeply respect their knowledge and as I've said, I learn from them on a daily basis.