[Kashuu is, in the end, one of the more insecure swords. In spite of his grandstanding and displays of confidence, it's really only his quick attachment that keeps him from being as unsettled by Midare's words as he could potentially be. They still cut, though - cut at deep fears that he likes to set aside and ignore, because it's too hard to do what he needs to do otherwise.

Maybe Midare went through something similar, but had been unable to neatly box away the same concerns. That's not a surprise, is it? It's not like he's unaware of the points that had been made in that furious spiel. The "lifespan" of a tantou isn't anything to boast about, and they're comparatively easy to forge and renew. But that doesn't necessarily mean their saniwa doesn't care about the ones that were lost...

But more importantly:]

Even if you believe that, what did you think pulling this stunt was gonna prove, huh? You wanna show us how disposable we are?

[Keeping his own blade pointed forward, he moves carefully while talking with the intent of scooping Midare's vessel up. That way, at least, he won't be able to hurt himself or anyone else.]