Nodes should only be propagating txs increasing the fee, but yeah, the possibility of spamming the network through increasing the fee by 1 sat steps seems to be worse.. But users can already do it with RBF transactions. Only ancient pre-RBF nodes would not propagate the tx.
Nodes do not propagate RBF replacements unless they pay a minimum threshold of additional funds per byte. The cost is high enough that it'd be a lot cheaper to just buy cloud servers and DoS attack the entire P2P network directly.
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