Honestly, I think the only reason we knew about Metroid Prime 4 already was because nintendo wanted to say "Yup! Metroid is a thing again!" Giving reassurance that it was even a thing was a bit necassary in the case of a franchise previously in such a dire state.
Personally, I don't really like waiting years for a game to come out. It's exciting at first, but then I just get bummed out and anxious when I realize I'm going to have to wait years to get it. If I saw super cool Metroid gameplay id probably want it now, not a year and a half from now.