Good metaphor and I understand your point, but I think we can be a little more exact with the language. What you are talking about is time. I would just slightly correct that all possibilities don’t “exist”, they are simply unrealized potential of superposed possibility. What you are attempting to narrow by choice is the timechain at your locality within it. As Bitcoin makes clear, UTXOs in the mempool don’t yet “exist” in the chain, they reside in a superposed state, self-referential to their prior state: valid, but unspent. They are constrained potential, not infinite—bounded by rules and history, awaiting measurement. Only when they are mined and confirmed do they become part of existence within the chain of time. The block, through the nonce, and the ledger together define the boundary of existence. The future cannot be written without the potential of the confirmed past.