It can, but gold is about the only thing (next to clean water) on our planet that is guaranteed to maintain its value (within a margin, of course.) All crypto-currencies are still incredibly susceptible to wild market fluctuations, and while unlikely could go to zero in an incredibly short time. Likewise, you're not dependent on the power grid or an online connection to trade your gold.
There are ups and downs to both - that's why it's imperative to diversify.
There is way more gold than we think inside the earth, just think that it was said (I'm uncertain.) that about 2% of earth's core is gold.
I would bet on us harvesting asteroids prior to the earth's core
Actually, not even "within a margin". Gold's intrinsic value has never really changed. It's the dollar that's been jumping around like a monkey going up and down (mostly down) depending on what new arbitrary round of market manipulation central banks have decided to do. Holding a gold bullion in your hands is still the odly greed satisfying experience that it has always been, and showing off your gold will still make your neighbours jealous and expose you to robbery, just has it has always been the case.
Gold is far from guaranteed to maintain its value, nothing is guaranteed in that way. Value is always based on perception. Diamonds were pretty worthless until De Beers began their massive campaign about diamond engagement rings, now diamonds are incredibly valuable. If someone started to diversify into something else away from Gold, crypto currency for example, then with enough weight others would move and diversify out of Gold and its value would drop massively. In someways akin to what will happen with the stockmarket, people diversify out to Gold so the stockmarket crashes and Gold soars. Something is only valuable whilst it is in demand.
De Beers has a (virtual) monopoly control on the diamond market. They're artificially restricting supply to keep prices high. It's not a fair comparison to gold. There is no single vendor who has a huge majority share of the sale of it.
Yes, value is always based on perception - but it's also based on supply and demand. Gold is actually genuinely scarce, not artificially (like diamonds). Likewise, historically - gold has always had notable value. Yes, there are no guarantees in life, but of all resources (next to water) gold will retain at least some decent amount of value. Sure, if there were a worldwide nuclear holocaust and we all went back to living like cavemen, it would likely go to just about nothing - or nothing. But, short of that, humans have always valued it to some extent.
Indeed as you say value is impacted by supply and demand, my point being that if enough of a perception shift was created then demand would drop in a very significant way as the demand was fulfilled by something else. Four years ago Bitcoin was worth very little, then over the course of a few short months it ramped up to over $1000/coin, it has been up and down since and is currently far lower than it was worth at the end of 2013. There was a high enough perception of value back then that people were willing to pay over $1000 for something that six months early would have been $2 or so. Things can change almost in the blink of an eye, things only ever retain value if enough of the population is sold on the idea that said thing is valuable. There are alternative to gold though people still use it because they feel it is better to.