What do you think about it?
It doesn't really explain any of the properties and design goals of Bitcoin and Bitcoin's inner mechanics to achieve those goals.
Your analogy merely describes in very very basic terms, and wrongly too, what a Bitcoin wallet is for. What bothers me is that you paint a wrong picture actually.
If the blockchain is the cave and all coins exist only on the blockchain then how would "the magic spell" (your wallet's seed) give you access to all the wealth in the cave (aka all the coins of the blockchain) as you describe it. This is such an utterly wrong analogy, tbh.
You have to invest a bit more brain greese to come up with an easy to digest but still accurate ELI5 description of how Bitcoin works and why it is as it is.
Key points to understand about Bitcoin for me are:
- no inflation like majority of gov's money printing habits (it has actually a capped and deflationary emission)
- no need to trust any centralized entity, it's rather permissionless and no-trust-needed decentralization
- anyone can participate in Bitcoin and needs no permission from any entity
- distributed, decentralized and immutable ledger
- no other entity can spend your digital coins
- digital coins can be spent only once
- you can't arbitrarily copy your digital coins and spend those copies
- you can't fake digital coins due to high mathematical and cryptographic security
- it's unfeasible to break security and steal others coins (not enough money, energy and time in this solar system or galaxy to be any successful if you dare to try)
I'm fully aware that's no ELI5 explanation and maybe it's not even complete, just the key points off the top of my head that I deem important to understand what Bitcoin is about. How to achieve the design goals is another story also worth exploring.