Sorry with the (kind of) stupid question, but if you have to constantly sharpen a sword, would you be left with no sword left after a period of time?
This is kinda like the old tootsie pop ads. How many licks does it take? Who knows, because something eventually bites it. You could, theoretically, grind a blade to dust, but it would break long before that happened.
So, when sharpening most blades, you grind down, or shave off, tiny bits of the metal to create a new edge. Thing is, the amount of metal lost is, usually, minuscule. Now, if you examine the edge, you will see signs of repeated sharpening, but you’re not going to grind away the blade.
A critical thing to understand about swords, and most weapons, is that it needs to be solid. It’s going to experience abuse as part of its normal function. Now, proper sword use will try to minimize that damage, but it’s still going to be suffering wear. Sharpening is one of the things you do to keep operational longer. Eventually, these things still have a useful lifespan, no matter how well they’re treated. Maintaining them extends that lifespan, but it doesn’t fully restore the weapon.
So, you can see where someone has been sharpening a weapon, and with enough experience you can make an educated guess for how much use a given blade has seen. You can also, sometimes, make a pretty decent assessment for how it’s been used. A blade that saw combat will look very different from one that was used in a handful of duels. A knife that was used for utility will look different from one that was used to parry attacks, (and I’m not even talking about design differences, I just mean the wear on the blade.) You can assess the skill of the armorer who maintained the weapons. Though, this goes beyond simply looking at the blade and wear on it.
If you did have an armorer who was grinding down a sword to keep it looking fresh, the end result would be that, in combat, the blade would simply sheer off. It might botch a parry, or shatter on impact. You need the mass of the blade to maintain the structural integrity, and when you start grinding that down, you’re trading the long term durability for an edge. Now, that’s something people do, but they’re not going to be grinding the core, they’ll be grinding near the edge of the blade. (One of the ways you can tell the age of a blade is by looking at how much has been ground down, and how sharp the angle is leading to the edge. )
Nicks are another good indicator. The blade struck something and deformed the metal to the point that it couldn’t be hammered back into line. Now, that may mostly sheer off on its own, or it might need to be ground down. Either way, that’s another consideration. Of course a nicked blade does, also, function as a guide path for an impact to damage the blade, and we’re back to, “you won’t see it grind away to nothing, you’ll see it break.”
Even if you did try to grind a blade away, eventually the core would break. So, it’s possible to do this, but the grindstone would eventually break the weapon. The only time you might something like this is on mock weapons used to train armorers. Where they’re asked to grind billets instead of working on actual blades. So, that could happen. You’ll also sometimes see this with older kitchen knives. A knife that’s been in use for decades or centuries can be whittled down to almost nothing. But, that’s not a combat weapon, and faces far less abuse as part of its normal life.
So, how many grindstones does it take to get to the core of a blade? Crunch.