All Japanese cast (at least for the major actors, they could probably be more lenient on extras in the background). Yeah, that’s possible.
The whole cast speaking Japanese the entire film? Err…..
I remember hearing countless horror stories from theater employees when Pan’s Labyrinth released. People coming back to the ticket booth and being all, “what is this crap, we have to read the film?! Why isn’t this in English, this is America!”
”Well, sir, the film is set in Spai-“
”AMERICA!!!”
Now the film still did well in spite of these dumbass motherfuckers, but the film’s budget was pretty modest. A film like Ghost of Tsushima though will have a much bigger budget and I doubt any studio is going to be willing to make that big a financial risk.
And no, generally people have not become more open minded about subtitles since then. Netflix reported 78% of their viewers watched the series Dark dubbed versus the original German version.