Which parts of his defensive abilities though? We've seen in the past how players can look excellent ok one system, then out of their depths in another. What we ask from our centre backs is quite different to what some other teams ask of them. I would argue a smaller number of games, but playing in our system whilst working with our coaches could be more beneficial than more games at a different cub.
You're right about KWP in that there are no certainties. We'll never know the answer to the counterfactual question, what if he was loaned out.
Meanwhile Pochettino has succeeded at Spurs and Southampton at bringing through young players without loaning them out. I would say succeeded at a better rate than any other Spurs manager I can remember, and better than most other managers in the game.
Perhaps we will start using loans more now that game time is less accessible due to the quality in our current squad. Perhaps. But I don't see it as an obvious answer for all young players with talent short on game time.