You're assuming that Persona 5 will be shown at the conference, while I think what others like A Dead Diehard are considering is that it wouldn't and that the trailer they would release outside of it would have subs.
I'm just playing armchair marketer, but I still think it doesn't make much business sense for such a critical darling to miss it. E3 might be losing its luster, but it still has more visibility than Atlus USA's YouTube channel would have.
While I personally don't mind the honorifics, your argument doesn't make a lot of sense when you give it some thought. I'm not really sure how to explain it, but I'll give it my best shot. English doesn't know honorifics. They don't just don't exist in that language. There is no way to accurately portray the nuances of a certain honorific in a language that doesn't use them. Accordingly, every attempt at translating them can only result in an approximation.
Japanese, as you pointed out, relies pretty heavily on honorifics to denote the social relationship between two speakers. You can also infer a lot of information about the speaker's self-perception from the way they use (or don't use) honorifics. However, I doubt that Japanese speakers actively think about what honorifics they use whenever they talk to someone. I believe it's similiar to German or French which distinguish between "du/tu" (you, casual) and "Sie/vous" (you, but polite) since that also reflects the connection two people share. It's also a better point of comparison than "Monsieur" (or "Herr", by extension) as it shows that there is some information in a dialogue that cannot be translated directly without sounding unnatural.
The point is that you cannot copy the characteristics of one language and past them on another language without dabbling into problems. You will always lose information during the translation process even if you chose to write around every single linguistic or stylistic problem you encounter. A Japanese native speaker considers honorifics as something natural while they don't naturally appear in English. Assuming that the characters in a work of fiction are native speakers and only interact with other native speakers with a flawless rendition of their native language, it would make sense to translate the dialogue of such characters with a flawless rendition of the target language.
That being said, I really don't mind the honorifics and I actually want them to stay for the sake of consistency within the Persona series.
This is why I have a distaste for them. I can deal and understand their uses, but I wish they weren't there.
As you said, the script exists to be natural to portray characters with a certain degree of believability, so that even if certain parts are stylized beyond reality, it's still grounded to keep the audience invested. At the end of the day, the story is one of people, all with relatable, or, at least, understandable issues. Each culture has their quirks, but they're still of people. Using a franken-language as your first line to connecting with these people is only going to put up barriers to emphasize the alien qualities, whether they're yours or theirs.
This is a lot more obvious in the range of manga fan-translations that translate everything literally (or at least, always make direct equivalents for certain parts of speech), but at some point, it risks losing sight of what the intent was. The words don't matter they're means to communication so idolizing specific word usages in the name of authenticity only undermines the credibility of the story. It further pushes whatever plights there might be from immediate and personable into the realm of weirdo, abstracted artifact. The languages aren't directly analogous, which is why some information will be lost in instanced cases no matter what, but the goal should be to have the same net information by the end. I think a lot of people get obsessed with the idea that these concepts are indescribable in English, when they're just expressed in different ways. It might not be as concise, or it might present a connotation minefield, but it's possible to navigate the mess.
I think they're kept in Persona translations to make them sound more like an anime that the target audience would be familiar with, not for this supposed extra nuance.
The thing is, I don't know any English-dubbed anime that keep the honorifics.