NSMB is a series that has gotten flack for being unoriginal, in stark contrast to the creative Super Mario Galaxy.
This is where you're losing me. Both of those games are Mario games. So... then, ambition isn't inherently tied to newness. One team made a Mario game that was unoriginal yet another team made a Mario game that was creative. He's the same Mario, red hat, blue overalls, and mustache. Ambition is limited by a development team's willingness to be ambitious. Replace Mario in those games to an arbitrary new character, call it something else yet you still have the same level of creativity. You're blaming the creation; I'm blaming the creator.
Of course that's what I consider Nintendo's golden age and to acheive that standard again they HAVE to make new IP. Super Metroid is great because it's the logical peak of the 2D Metroid formula. You can't make a better 2D Metroid unless you do something different with it like Fusion did (and later that other game but we'll ignore that one). Ocarina of Time was the logical peak of the Zelda formula and that's why Twilight Princess got a lukewarm reaction. Twists on the formula like Majora's Mask are the only direction they can go. New IP is the easier way to maintain the high standard of Nintendo game design and it doesn't carry the risk of fan backlash like twisting up the existing franchises does.
Only a Sith deals in absolutes...
How are you coming to these conclusions? Of course some developer can make a better Metroid game than Super Metroid. The IP isn't holding itself back. Creativity or the absence of it is. I don't see the connection between ambition and age. You can be ambitious with something new and you can be ambitious with something old. Find someone who's passionate about the material and they'll make you a great product.
And how do you figure that Twilight Princess got a lukewarm reception? In general? How do you plan to prove that? From the media? Pretty sure that's wrong. From you? Me? I didn't like the last 25% of the game, but I'm fairly confident that I'm in the minority of that belief. I didn't like Skyward Sword overall. Probably in the minority thewre too. Again, not for you ≠bad.