I could broadcast a second transaction that has the same inputs but a higher fee, without marking the original transaction as replaceable. Miners would most likely favor the second transaction and reject the original one. Why should I mark my transactions as replacable if they can always be replaced by broadcasting another conf-0 transaction?
I could broadcast a second transaction that has the same inputs but a higher fee, without marking the original transaction as replaceable.
Default node behaviour is to accept the first-seen transaction into the node's mempool. So your second transaction that spends the same inputs would likely be dropped by most nodes, and thus it would be less reliably broadcast across the network, and therefore be less likely to be included in a block.
Of course, the second transaction could still be included in a block, but this 'first-seen' preference makes it less likely. Even though a miner may prefer the transaction that pays higher fees, if they do not hear about that transaction in the first place then there is no chance of it confirming. An explicit RBF marking allows the user to more reliably broadcast a modified transaction spending the same inputs later.