The 'trio' of characters being largely safe from anything major in the plot from affecting them negatively is a well-worn shonen trope. Nobara getting offed like that was to showcase the stakes the author was willing to play with. It almost never happens that primary cast members in a Shonen trio get offed.
But offing, or otherwise sidelining, the girl in the shonen trio is not the trope-smashing move he thinks it is. It's the trope itself. So it's a stunt with no real purpose. Plus, he already did this with Yuji 'dying' but surviving and training unbeknownst to his friends.