Shanks had to develop a fs killing technique to deal with mihawk.
Zoro will need to do the same, also swordsmanship is tied to haki, as the pinnacle of all swordsman zoro will have the pinnacle of haki as well.
Shanks having a FS killing technique is not canon.
And it being developed because of Mihawk also isn't canon...
And even if it were true, just because Shanks needs something, doesn't mean Zoro needs it too.
You can have the pinnacle of all haki without having FS because it isn't the only advanced branch of CoO...
fs is the perfect ability for a swordsman, zoro will have all 3 forms of adv haki by eos, fs is also perfect for Zoro since he is calm and stoic
He already has advanced forms of all 3 hakis...
FS would make Zoro arguably weaker because it makes the user stop sensing the intent.
No need to play a role of Sanjibros and wish for something just because someone else has it too.
Zoro is walking his own path instead of fishing abilities that others have.
It is Zoro who has unique abilities that nobody else has.
Not the other way around.