Users expect determinism and reliability out of the Bitcoin network. There should be no features which rely on the goodwill of node operators.
Exactly, but that's not bad. Merchants and customers should rather rely on the good will of their between relation, not on the Bitcoin network. If a customer is malicious, it doesn't matter much if most nodes choose to not propagate transactions that double spend an RBF-disabled transaction; he's surely discouraged to try, but there are alternatives such as miner bribing.
On the other hand, if there's a good relation between the merchant and the customer, regardless of whether the transaction opts-in for RBF or not, there won't be double-spends. And I don't know about you, but I mostly have good relations with those I transact with.
Question: how hard is it to full the network with nodes that have Full RBF? Could you setup multiple full nodes in one machine without anybody knowing one person is running all?