it's the matter of what is default - choice to switch-off RBF is good, as well
especially for newbies, they may even don't know wtf is RBF - till their first transaction frozen in mempool "forever" (that was common case in 2017)
In case of default RBF - there wouldn't be such not easy to solve surprise
you cannot switch-off RBF if is enforced on the node that will broadcast your transaction. you are taking away my choice of not using it.
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