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Which means that existing users who choose not to participate in meaningless transaction spamming see their share of the market cap diluted, so they are still paying for it.

Existing users might also lose their ability to use the blockchain. It almost seems like there should be a minuscule fee to prevent malicious spammers from trying to crash it.

Well, we have bandwidth, but we can see that it creates some problems. I'm sure they're going to try more novel approaches in future HF's.