The operation has an obvious connection to the WG and more importantly the Celestial Dragons. Don't be surprised if Im himself is immortal or the Gorosei.
Law also has the whole D storyline left. Those are way bigger things than Kaido at this point
Which is why he needs to Die in Wano, because he will entrust his Dream to Luffy
It's no different than Pedro & his Dream
Oda introducing it just so that one day he tells us a Previous User used it on Im-Sama is Lackluster
Fans obviously want to see it being Used, not just hear about it
And ofc Law's Dream is way bigger than Kaido but his Role isn't Bigger than Wano Arc
This is Conclusion to the reason they made an Alliance in first Place, it's the Finale of Law's Story that started since Punk Hazard
Actually after Kaido has fallen, Law & Luffy's Alliance is basically finished so if Law survives, his Role & Screen Time will get reduced significantly so Oda bringing him back for Eternal Youth Operation will seem like a drag
Law's Story started with Punk Hazard & it should End with Kaido
He helped Capture Ceasar & then he helped Defeat Doffy so now he should Sacrifice himself so that Kaido is Killed
It's a fitting End for him in this Wano Saga