Sir William,
I concur with you that the prefix and name field help text could be better; I would like to see it do a better job of explaining the different display results.
In the meantime, prefixes are used. As has been explained many times (but is not yet made clear in the referenced help text), prefixes are displayed differently depending on the context. They are not displayed on the profile page header but they are displayed on the subsequent line of the person's name. As Chris W. has already explained, much discussion went into the resulting display options. And I see no evidence it's likely to change.
Because you have chosen to disregard wikitree conventions for use of the first name field by placing your title there and because you've also placed "Sir" in the prefix field, your own profile page currently displays your name as "Sir Sir William Arbuthnot" which frankly looks pretty silly and not at all in alignment with the respect your title seeks to convey and receive. I think you do yourself and your title, sir, a disservice by continuing to press this issue.
As for the use of Mr and Mrs, I think we are not going to achieve consensus on that one. Just as you may feel strongly about the use of Sir, others feel strongly about conveying respect through the use of Mr and Mrs for both the living and the dead. My own opinion is that if a title helps distinguish different people by the same name or conveys historically significant information, then fine. But I don't think either you or I are going to "win" our arguments on this topic. I choose to focus my energies elsewhere.
With all due respect,
Jillaine