Shanks losing an arm is fine

[Warmmedown]

Yeh he's strong already, but it's not easy to move fast across the water, grab someone and then swim away.

 

[SSJ2]

He might have done it intentionally.

 

[Warmmedown]

That would be weird. Because he knows whoever eats Nika is special?

 

[SSJ2]

If he's aware of the fruit and Luffy seemingly inheriting the will of D, maybe he wanted to inspire him.

 

[Future Warrior]

It was definitely on purpose. It's easy to forget but in One Piece, human skin can still be pierced by fodder bullets and swords if Haki isn't wielded, top tier or not.