When there are technical experts stating opposing things it is hard for people like me to know what is true, Mike and Gavin certainly do think that it does reduce the functionality of zero confirmation transactions.
Mike and Gavin have not commented on Opt-in RBF as far as I am aware; you are confusing his comments on full RBF. But on what basis do you believe their expertise is greater than a dozen other parties?
Opt-in RBF is explained in some detail in the
FAQ on Reddit, linked previously to you in this thread; you say that you are open to other views, but you seem to have made no effort to familiarize yourself with them.
That is peculiar considering that he has spoken out against it on twitter, I was not aware that he had not told the Core team about his concerns, I find that a bit strange, maybe you have a better insight on why this is the case. I am going to ask Gavin myself why he did not speak out against RBF when it was being implemented into Core, since I do find that rather strange.
I just checked his twitter feed and can find no comment on it in the last several months. As far as insight, Gavin is not very active in Bitcoin development and hasn't been for a significant time, before the recent blocksize drama. That he didn't comment on the PR was unsurprising on that basis, but he certainly was aware of it-- since he asked when the merge was being discussed in the weekly meeting if we'd talked to any other wallet vendors about it (though this also indicated he hadn't recently read the PR).