Jim Posen suggests that using a 0 relative locktime could enforce RBF on the spending transaction, but Marco points out that if the parent is RBF, the child inherits it as well. However, Matt Corallo points out that blocking RBF with a large-but-lowball transaction is possible, citing BIP 125. Peter Todd notes that preventing bandwidth usage denial-of-service attacks on the mempool is why the rule exists, and dropping it would allow attackers to repeatedly broadcast and replace transactions to use up tx relay bandwidth for significantly lower cost than otherwise.