Which is different than the last two or three games in each franchise, how?
It depends on how good the game is and how good it looks.
Metroid has one anomaly: Other M. And that game is a brilliant case study of how to fuck around an existing, loyal fanbase.
Prime is probably sitting on 2+ million worldwide. Echoes underwhelmed, as I don't think it broke 1 million. Prime 3 last clocked in at ~1.3 million worldwide. Given the cost of production, age of the franchise, and demographic of the Wii, this is not bad at all. Metroid is, for all intents and purposes, a fairly consistently strong selling franchise within Nintendo's realm.
Other M fucked it up due to Nintendo having the bright idea to sculpt the series into something it wasn't. In a foolish attempt to appeal to the cinematic/story driven market, they took a franchise unknown for these themes and butchered it until it fit the new mould. Surprise surprise, it sold like shit. New audiences didn't want the game due to the subpar quality, poor marketing, and iffy Wii demographic, and Metroid fans were pissed off that the franchise had taken a new direction.
If anything, Metroid's design and sales history shows how much potential the series has, and how dedicated the fanbase continues to be.
The decline in sales for Star Fox is proportional to the increasingly shitty quality of the games, and abandonment of the core design philosophy, which much like Other M conflicts with the established fanbase. Star Fox Adventures? Zelda clone. Star Fox Assault? Junk game. Command was a weird attempt to return to roots, and it didn't work, indicating the franchise probably needs to be taken back to the drawing board. A case for SF64 3D can be made to support this, but that game tanked for a multitude of reasons.
So looking at this, why on earth would Nintendo want to
combine the franchises, when seemingly everything written there is what
Star Fox needs, and not
Metroid? Ground exploration, combat systems, character banter, space flight and battles, and so on. Sounds like a recipe to reboot Star Fox with a big blockbuster game and hope like hell it pays off.
But Metroid doesn't need it, not when we've already come off the back of a dud that shows just how fickle the fanbase is. Again, Metroid isn't example of a series that has worsened in sales. Prime 3 is a current generation title that made ground lost by Echoes. This is
growth. Other M distanced itself from the franchise' roots, and ended up tanking. Metroid doesn't need to be merged with Star Fox in an entirely new cross franchise game to be 'revived'. Metroid needs to go back to being Metroid.
Valva ripped id off! That's the story of Quake III Arena. Suesuesuesueblarfggehrrg!
Nope. They're part of the same universe, don't you see? Equally profound philosophical mumbo jumbo on the grim dark horrors of war.