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I like the wind of subsidy in early tx's sails analogy. But the recent appearance of "cosmic background spam" makes it outdated.
The problem is that sailboats are expensive to construct and operate, while Bitcoin tx are not.
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Yes, they're not expensive, especially the spam ones. And guess why is that so ? Because there's no room for those transactions in a 1MB block, and whoever is sending them can just resend them over and over again without actually spending a dime on them.