In my opinion, it all depends on all sorts of possible factors.
First thing will be the collectors themselves, and what they personally are seeking out.
Are they looking for the item to be as close to being factory pristine as possible, or are they more likely to want some battle scars on the items they acquire? That's a big personal taste thing, and collectors come in all flavors.
One collector would love to own a battle scared Colt manufactured Thompson Submachine gun, while another collector would rather have a pristine Colt Tommy Gun still in original factory packaging.
There is also provenance... If the item was owned and used by a famous person, (in battle or otherwise), that will usually add big $ to the item. There's a German made 'Congress' pocketknife that Abraham Lincoln had in his possession when he was assassinated. It's currently housed in a National Museum, but if it were held by an individual... That person could fetch crazy $ by selling it at a reputable auction house
There is also general scarcity, even sometimes rare items, which can add big $ to them. And, if rare, (or at least pretty scarce), even an item that may not be in all that great of shape, will retain some decent value, since the collector may not come across another any time soon or even ever again.
When it comes to military knives, usually a ton more wind up being issued and used, sometimes very little, and sometimes abused), by the soldiers.
The ones that somehow wind up not issued, and later acquired by the civilian market, wind up being the more scarce specimens. That usually does cause them to be valued at a premium... Nature of the beast.
There is desirability... I mean, even if something is old and in nice shape, but has little collector following... It's value will usually be fairly low.
There is another big elephant in the room, which is that collector interests can wane and disappear over time.
What was collected by one or more generation of people, can become extinct as the interest for those goods may become non existent with newer generations. An example is the antique wood furniture market, and how it has dropped considerably over the past so many years. Younger folks seem to have little interest in "that stuff", so as folks pass away and leave behind their belongings... Less people will remain that desire "that stuff". Sometimes the surviving family will simply throw that stuff into the dumpster, or sell it off in bulk to an estate buyer. But, with the desire for such goods having waned so much, it may eventually wind up being sold at the local flea markets at a fraction of what they once cost. The "old stuff" is just not seen the same way by most of the current crop of younger folks... They are bringing in a new era of big change from what may have been the norm.
Same with Silverware... Most new generations don't want it, and are likely to just sell their inherited pieces off as scrap silver metal that gets melted down for cash.
In other words, while something may have been collectable to one or more generations of people, it only takes one upcoming generation to pretty much squash that tradition of collecting them.
There are so many more variables to consider, and so it can be a complicated thing overall.
Hope I've explained my position halfway decently
