I disagree with your comment regarding H.S trumpet players still playing their 7C and it being old school thinking. I played professionally in orchestras on a 7C and no one ever complained my sound wasn't full or broad enough. Some legendary trumpet players eg. Bud Herseth played on 7C's. Rafael Mendez played on very small mouthpieces if my memory serves me correctly. I think the trap is to feel you need to move up to something larger using the bigger is better principle, but I agree with your point about a young player moving onto a small lead piece and not maintaining practice on a more regular size mouthpiece being a mistake.