If we were going into the season with the lad as fourth choice centre back, which I think is the scenario we were discussing, he would have to be part and parcel of the first team training set up. I think it would then be difficult for him to become a regular fixture in the U23 starting line having not trained or done any match specific tactical stuff with the U23 manager and the rest of the squad.
Also, you never know when he will be required for first team duties. I wouldn't normally expect us to go into a first team game with two centre backs on the subs bench, but we may do so if there is a medical question mark over one of the players, or indeed he may be the actual bench cover in the event of injury or suspension.
I'm not arguing with your assessment that young lads like him learn more from playing competitive games, I just don't see how this could be done in those circumstances.
It's why the decision to keep young players with the first team, or send them out in loan, is such a difficult one to get right. Ideally, for the player and not necessarily the manager that is, a senior player in his position will get injured thereby giving him an opportunity.