It’s been a franchise since Splinter of the Mind’s Eye. The word franchise is also utterly unrelated to the word saga other than them both being used for media. And if you think 1978 is an example of ‘modern marketing’ then that’s an… original take.
If you think the presence of interconnected characters ‘ruins’ stories then maybe you want to only watch or read stand-alone media rather than anything with a sequel, prequel, or spin-offs… you know, like Star Wars.
Luke being the only hope against Vader is a mythical thing because Star Wars is a fantasy. Not literally that he’s the only Jedi physically left. Luke (and obviously Leia) are the ones that can defeat Vader because they represent the good in Anakin. It was never about physically being able to defeat him else Yoda would have done so, so Ahsoka, Ezra, Cal, or any number of Jedi knocking around is completely unrelated to the story of Vader’s redemption.
Luckily Ahsoka is there to tie together the much more interesting parts of the saga than ‘whining farmhand tries to make genocidal tyrant good again’ and long may that continue.