We already seen him fight in Wano after losing an arm. He showed nothing new whatsoever, and both he and Law went all out.
Aehm... what?
We have seen him, in the story, fighting 3 times:
1. Before TS, his first appearance, so basically everything was new.
2. On roof against Kaido, there he pulled new things like Punk Rotten, Punk Vise, Slam Gibson... moves he never showed before.
3. Fight vs Big Mum, there he pulled new moves like Punk Pistols, Punk Corna Dio, Damned Punk + Awakaning
Where exactly did "showed nothing new whatsoever"?
That is constant growing and showing new feats. And yes, to beat an Yonko, they both went all out, but... whats the surprise?
Law hasn't done anything new either since Wano, except for using awakening faster (he was more rested) and knowing to use haki against a devil fruit (not the first time he used haki though). And Law is not winning against Blackbeard either, you need to get your fanboy heart in order.
"Has nothing new" - "ussing awakaning faster and knowing to deal with devil fruits" ... you get it yourself right? He is freely using awakaning right now, which is huge. Comparing with Luffy, he now is G5 using easily and instantly, which is also a power up. Claiming this is the same strength by Law as vs Big Mum is again a strange take.
Additionally, we have seen nearly nothing from the fight so far. We got the initial clash, thats it. Dont expect something else from any fight in the series anyway.
Nope, for Shanks to be there when Luffy becomes a greater pirate, he has to be involved in the chase for one piece as well. He is finally on the move, if you think this is where he gets defeated after finally doing something, you're gonna be very disappointed.
So you buy this Shanks is suddenly going for the One Piece, when his all story claim is that only the one Roger is waiting for will get the one piece. When Shanks whole story was about Luffy finding the One Piece, but suddenly he is for it to? While he never ever before made one step in that direction.
But ok, even if he does, his going for the One Piece has nothing to do with Luffy at all. Also become "a great pirate" is not connected to Shanks at all. Shanks can lose and literally nothing will change in direction of Luffy.