However, it also essentially stops any business from accepting zero confirmation transactions for small values, since all transactions (even those opted out of RBF) could potentially be replaced using RBF.
That's unfortunate, I do like the occasional zero confirmation transaction that's instantly accepted.it will also help the mempool to be less congested as some people will be able to pump the fee and increase the chance for the unconfirmed transaction'to be included into a block.
RBF doesn't empty mempool, for every transaction that gets into a block because of the increased fee, there's another transaction that doesn't make it anymore.~ I'll have no idea which one will actually get mined until one of them is mined?
Isn't that the same now? If you use RBF, there are 2 valid signed transactions out there, and it's up to the miner to choose which one to include.For brick and mortar shops who've been accepting bitcoin already I don't think this is a big deal as most people tend to be honest when dealing locally and face to face. For online retailers it may be an issue if they're selling digital items that customers want to download instantly. As LN becomes more and more common, it'll become a non-issue.
I admire your confidence in people's honesty 
And even though I agree most people would be honest, I fear this would attract dishonest people who see an opportunity to scam a shop.