I don't think it really makes sense for someone to create a low-priority transaction that is not RBF today.
It doesn't, but a lot of people use wallets which do not support opting in to RBF. In such cases, users are forced to broadcast with a high fee unless they are prepared to wait for however long it takes for 1 sat/vbyte. In a full RBF world, such users can broadcast with any fee they like, with the knowledge they can later bump said fee (using different software if needed) should the mempool suddenly start filling up as it often does.
These businesses often effectively require customers to pay a next-block transaction fee. If a customer is willing to pay this high of a fee for ~instant acceptance by the business, they would presumably be willing to pay a similar fee for next-block acceptance.
But conversely if these customers want to continue having instant acceptance, then they will move to Lightning and can instead open Lightning channels at any time they like in advance, which most will obviously choose to do when fees are low.
I would think that most miners probably use multiple nodes
Almost certainly, especially considering the low requirements needed to run a node when compared to the requirements of a large mining pool. And as I said above, once you start running multiple nodes, it becomes very easy to learn about full RBF replacements.