Yeah this was definitely something else that popped into my mind. As much as I personally would like Nintendo to do an on-stage conference it might not make sense if they're holding back on announcements since they're going to have to wait things out until they release new hardware in a couple years or so.
i think the stage conferences have an additional problem, and that's if things get shuffled around at the last minute, the results can be really shitty. i believe that's what happened to nintendo in 2005 when they spent a good long fucking time talking about electroplankton. it's not instantaneous work, but it's a lot easier to edit out a trailer you decide you don't want to reveal from a video than cut it out of a live presentation.
and really that's what these things are becoming anyway- presentations. in the past, and with smaller companies, press events are usually trailers and some info like release dates about games, maybe some partnerships and maybe some sales figures. it's not meant for the general public- it's meant for the press to tell to the general public. like, the entire reason of having a press conference was so you wouldn't need to put on a whole show for everyone to see because the press was going to do all your work for you. now these things are streamed and broadcast on television and they really don't have the same feel they, now that companies are increasingly aware people are watching. but even so you get stuck with things like people talking about tv for 10-15 minutes (sony), and/or bad attempts at humor (every third-party ever). if all you're going to do is a show, then from a conceptual level, why does it have to be live and what are you gaining that you couldn't do from a tighter pre-filmed show? that's not a rhetorical question either. these are things that should be considered when planning a show.