Technically Young was actually Master Xehanort in the DDD fight, so I guess he just has the skill.
Sorry for being the Negative Nancy, but the Keyblade transformation was something that I found fascinating when Terra and No Heart did it, it felt so... imposing, and terrifying.
Now Sora uses it and it looks like the Bros Attacks in Mario&Luigi.
Just from a """lore""" point of view, I'm a bit displeased.
Not everything needs to be backed by lore, but yeah, I get your point. As the series how progressed, the Keyblade has lost that "mysteriously magical and intriguing" charm it had.