Glad you’re not my financial adviy
There is a sense that this is true because of the ability of countries to print fiat currency at will, which drives inflation and upward pricing.
For example, an M4 sold new for between $500-ish in the 1970s would command an equivalent of about $3500 today. You get a sense of how well these have held their value when clean M4s sell for upward of $2500.
But that's not the whole story. The true value of something is what people will pay for it, driven by the demand viz the supply. That's how Leica gets over $6000 USD for a new M-A (the closest camera to an old M4). Leica figured out how to turn their stuff in Veblen Goods that are markers of affluence, thereby morphing into a lifestyle brand. People will pay those nosebleed prices just to "be in the Leica club". It gives them a sense of exclusivity.
In and of itself, this isn't speculation, it's social marking. BUT ... some/many people (and me) will buy relatively rare items like a pristine M4, M5, Visoflex, etc. on the speculative assumption that we can use it and either break even on the way out or even make money.
The problem with that theory is that I never want to sell my Leicas ;( It's captured inventory.
In general, if markets are honest and transparent, they are very efficient at pricing things based pretty much on the buying power of the currency in question maped onto the supply/demand curve. This all goes to hades in a handcart when there is either fraud or someone starts howling about "unfair pricing" and gets someone - usually the government - to interfere with market pricing dynamics.