It [Opt-in RBF] reduces the functionality of zero confirmation transactions.
Good! zero-conf was the definition of stupid when I got interested in Bitcoin, and with good reason.
I would like to preserve and strengthen zero confirmation transactions, like XT and Bitcoin unlimited have stated. There is a fundamental disagreement of believe here.
If I am a merchant should I reject RBF? Since if I did do this could cause other problems as well. Since it is the person that sends the transactions that chooses to opt in not the receiver.
Transfer your Bitcoin (in the unlikely event that you have any) to a wallet service which guarantees not to do RBF then and let them interact with vendors. Or just use ACH/PayPal if you like it so much. Many of the rest of use have use for something different.
This proves another one of my points, the solution according to you as a merchant is to transfer my Bitcoins to a third party wallet service or stop using Bitcoin completely. I would not consider this to be an "improvement".
I operate a brick and mortar store which accepts zero confirmation transactions directly, I would not like it if I would have to stop accepting Bitcoin or use a third party, which would mean I would then need to take down the Bitcoin sign I have in my store, I would still accept other cryptocurrencies of course. The blocksize would also be a factor in this, I do not agree with the path Core is taking Bitcoin into, fortunately we now do have alternative implementations to choose from and more are also now starting to appear.
These alternative implementations are critical for the freedom of the Bitcoin protocol. Alternative implementations represent alternative choices for us to choose from as a community, this is a good thing, without alternative implementations it would be an election with only one option, that would not be a choice at all.