Well they have to sell the game still, right? Is there really much point in using the "Legend of Zelda" name if you don't even have the main character in it? I figure a Zelda game without Link would be unmarketable.
Probably about as unmarketable as a Metal Gear game without Solid Snake or a Castlevania game without a Belmont. Neither of those things have ever happened, right? Come on, man... Snake Eater features neither Solid Snake nor a Metal Gear. "Oh, well, Big Boss is pretty much Solid Sn..." NO, HE'S NOT SOLID SNAKE. Metal Gear Solid Rising apparently stars Raiden exclusively and if the ending of Metal Gear Solid 4 is anything to go by and Konami doesn't reboot the series, we're probably not seeing Solid Snake suit up again either (unless they do some more **** with nano-machines or something...).
Alternatively, Other M stars the series main character, is called Metroid, includes many of the series signatures like missiles and different beams yet it's the least Metroid-y Metroid in the series. Say what you will about the quality of the game, but none of the previously mentioned things, namely the presence of its main character since you brought it up, changed anything. Point being, having this character or that isn't what makes a series, especially when the fundamentals of its themes are curb-stomped beyond recognizability. Other M AKA Are-You-There-God,-It's-Me-Samus turned our favorite bounty hunter into a stupid asshole.
Is there really much point in using the "Legend of Zelda" name when Princess Zelda doesn't even show up? Link's Awakening is essentially The Legend of NOT-Zelda because she's not even in the game. Furthermore, the entire set-up for The Wind Waker revolves around the consequences of Link NOT appearing. Does Nintendo have the balls to pull off an Empire Strikes Back-esque ending? The point is something is only mandatory if you make it mandatory. A Zelda game without Link isn't inconceivable because there's more to the series than the characters that make up its world.
What if Nintendo told the origin story of The Wind Waker's flooded world? For the purposes of this example, let's say the player takes the role of a
girl, later revealed to be Princess Zelda who FAILS to stop Ganondorf then her father, King Daphnes Nohansen Hyrule, begs the gods to flood the world. Would you call that game not a Zelda game just because it didn't feature Link? Narratively, it takes places in Hyrule and fits snugly in the series loose canon, explicitly tied to one of the main games. Thematically, a lone hero is tasked with saving the world from the forces of evil (what every Zelda game is basically about). Nintendo could do a lot of things with that set-up. They could take the easy way out and just outfit Zelda with things Link traditionally picks up. Or they could make a game in which players had no idea what would happen next, yet it would unquestionably be a Zelda game. What if Nintendo didn't reveal it to be the prequel to The Wind Waker before the game's release? At the end, we watch the ocean flood the land of Hyrule. Jaws be dropped, bricks be shat.
That sense of unpredictability is part of what's missing in Zelda today. In A Link to the Past, Ganon made his second appearance. Who knew? That was cool 20 years ago. Now, it's expected and when he shows up, I'm like, "This motherfucker AGAIN?!"