With full-rbf, who saw what transaction first doesn't matter: the higher fee paying transaction will always(*) replace the lower fee one. With opt-in RBF, spamming the network can beat out the alternative.

incentivised predictability is critical when designing low level protocols, like bitcoin. the knock-on effects of deeper, network-wide predictability are likely beneficial in ways that are hard to predict. for example stuff like the "sabu" protocol might even work if full-rbf is the norm.