The main thing that got the mempoolfullrbf option added to Bitcoin Core was the fact that full-RBF mitigates a variety of double spend attacks on multiparty protocols like coinjoin.

Basically, double spends are ok in multiparty protocols. But what isn't ok is people double spending in a way that causes transactions to take a really long time to mine. Full-RBF is a major step to preventing that, because it forces the attacker to use a transaction pinning attack, which is expensive.

Transaction pinning is also fixable too: https://petertodd.org/2024/one-shot-replace-by-fee-rate