Facebook went 10x, from $50bn to $500bn, after it was added to the S&P500. A similar pattern can generally be seen for other stocks being added to the index. That's one company / stock.
There is only one Bitcoin. There is only one Ethereum. You have investors globally buying mostly those two assets. Want to buy the asset class? You buy one of those two (or both)
Pension funds manage around $300tn. If those allocate 1 or 2% to crypto, a sparkly new asset class, you'll have a $3-6tn market cap. Which they might, given the above average returns it's been generating.
Gold is worth $8.2tn globally. Bitcoin in my view qualifies as digital gold. Why was gold originally picked as a store of value? You could've picked other metals. Or diamonds, etc. But someone decided gold was the one. Gold in itself isn't very useful. It's been given its value by people, because they believe it to be a store of value. (I'm not a gold bug and I personally don't think gold is worth holding) But think about this: how high could gold and silver go in a speculative bubble? $13tn? $16tn? Higher?
There will be 20-30-40% corrections along the way, but in my view, it's going much much higher -- very much like the .com bubble (which went up to approx. $5.6tn). Wouldn't be surprised if this entire space achieves a multi trillion market cap ($2-4tn) over the next few years, with most of that ending up in Bitcoin and Ethereum.
Note: this is not investment advice. Do your own research and make up your own mind.