(From what I've been able to gather, and I could be wrong) Twilight Princess sold better than Zelda 1, and was just as linear/structured as the likes of "Super Mario Galaxy 2" and was also quite narrative heavy.
Twilight Princess sold during the era when cinematic games were starting to really break out (i.e. cutscenes and story were huge selling points), and I really doubt most people bought it with the understanding that it'd be a supremely linear game.
Also, Twilight Princess sold during an era where videogames were much, much more popular generally than they were in 1986. In 1986, Zelda was one of only a couple games that had ever broken 6 million LTD; in 2006, that crowd was much larger; since then, 6 million is expected for a franchise like Zelda coming from a publisher like Nintendo.
Zelda 1's 6.5 million sales are worth much more in the scope of the industry in 1986 than Twilight Princess's 8.5 million sales were worth in 2006. 8.5 million is definitely a good landmark for sales that proves that the sales decline the series saw with Wind Waker and others was the product of a vision that didn't match the tastes of players. That is, it shows that Zelda can grow in popularity in a by the numbers sense.
But Zelda is no longer the biggest fantasy franchise on the block. It lost that merit years ago.
In addition to that, Zelda 1 was an early experiment that was never meant to completely define what Zelda would become; the two sequels that followed it became a lot more structured, and obstacle course-centered. ALTTP basically laid the foundation for the design structure that would dominate the series all the way up till Skyward Sword.
I'd argue that Nintendo in general fails at executing on good ideas beyond the first couple iterations, and on top of that they never have any long term vision for any of their franchises - and certainly didn't plan any games that way in the 80s.
I'm not sure why you're desperately running with this "Zelda waz more successful when it was an open world action-rpg!" narrative because it just isn't true.
We don't know, because Nintendo hasn't made it one of those since the 80s.
We do know, however, that Zelda's relevance is fading while other open world/fantasy/WRPGs are flourishing, that the Zeldas that have sold well have all been closer to the cutting edge of the kinds of video game worlds that have been presented in action-based games, and that Nintendo sees the solution as being to make Zelda an open world game.
I think that if even Nintendo can admit that Zelda should be an open world games, maybe the fans should too.
Oh also, lets not forget the fact that ALBW- the most open and action-y game since LoZ1, also released on a successful console- didn't exactly light up the charts; it's apparently hovering around the typical 1-3 million figure that LoZ games usually muster up (despite the well-deserved reverence the series has among enthusiast Zelda is kind of B to C-tier from a sales perspective)
I'm of the opinion that ALBW is extremely overrated and deserved the sales it got.
The similarities you've pointed out in your post are shallow aesthetic parallels equivalent to the mock "Mass Effect and Metroid both have shooting, power armor, upgrades, aliens, and spaceships; they're totally similar!" argument. It's just dumb.
They are?
I'm arguing that Zelda and Dark Souls both have similar DNA as video games, not that they should be compared because of superficial aesthetic similarities.
I'm not saying that they're identical, either. Dark Souls is definitely far grittier, also takes some cues from Castlevania in terms of world design, and builds RPG elements much more extensively into its systems than 3D Zelda ever tried to.
But in terms of the dynamics of play - the idea that your reflexes are the primary determinant of skill, that you wander into a new area and you might get turned back because you can't beat the enemies, that you can find a new tool that will be really helpful against specific kinds of enemies, that your character is basically dropped into a chaotic world with very little direction, and even the level up system that's present in the game - all of that is right in line with Zelda 1 and Zelda 2.
Even though its world design parameters are way different, Dark Souls scratches the Zelda 1-2 perilous adventure itch better than any Zelda game has since Zelda 2.