Would be great if this could actually touch anything near 5mil. I'll say 3-3.5mil, since they're doing a decent job promoting the series, but there's still certain people that will never be sold on the general concept.
From what I played so far Pikmin 4 has made quite a few noticeable changes. Probably the most changes for any game in the series. We'll have to see how that affects the feelings of previous fans and how it could attract new fans.