Is it that far-fetched to believe that Sony simply wanted to ride the Monster Hunter name in Japan while at the same time Capcom wanted to take advantage of the PS4's Western success to expand the series, hence the new direction? I understand the contradiction some users are pointing out but maybe it was overlooked to secure this deal. If true, the people who made this deal clearly thought Sony's goal of increased Japanese sales and Capcom's goal of series expansion in the West could both be met in one game.
Why would anyone in Japan buy MH on PS4 when there's traditional MH on Switch?