I'm wondering how long until Guts needs a new sword because he's hit something that it: A) can't cut, and B) is durable enough to shatter it.
A several hundred pound mass of iron won't just shatter when it hits something. On top of that, the Dragonslayer's been enhanced to cut through magical stuff. Guts isn't gonna need a new sword anytime soon.
If anything, I could see it getting augmented if I run into a good enough smithy.
When I say 'anything', I mean 'any of the numerous things in the universe that are probably harder than Apostle flesh'. Much like taking that thing to a cliff face: it's going to get wrecked sooner or later, even if it deforms before it shatters. Though, admittedly, iron is a lot less brittle than steel.
Apostle flesh is actually quite durable; most mundane weapons have a hard time hurting them. When Guts took on his first few bottom ranked not even apostle demons, a normal greatsword wasn't enough; only the Dragonslayer had the mass necessary to cleave through it.
Actually I can probably find Guts cleaving through rock and other hard materials without scuffing up the Dragonslayer. In fact, I recall he sliced through the hide of a dragon made out of crystals in one chapter.
Oh? I've had trouble getting past Rosalie. Up to which point, he was just utilising its normal properties: the edge might not've been that sharp, but sheer weight would cut through almost anything. By that standard, smashing it into rock would chip away at the rock
and wreck the blade utterly.
Of course, he didn't have the Dragonslayer for the Band of the Hawk era, and Rosalie's right after that. The whole Black Swordsman mess beforehand is pretty meaningless. XD