But why is it a *must* that Shanks has some power that propels him past Mihawk?
Because Mihawk was already the WSS, he's not striving for the title anymore and is waiting for Zoro to challenge him.
Shanks became a Yonko 8 years ago.
"Mihawk would win in a swordfight, but in an 'anything-goes' scenario, Shanks wins."
Shanks doesn't spend his time mastering his swordmanship, just like King, they have other means of fighting. Zoro, Mihawk, Vista, Kaku, etc. spend their time on their swordmanship.
Zoro trains his body and 3 sword style plus CoC now because that's his swordmanship, Mihawk probably trains his CoO and master Yoru, Vista and his flower 2 sword style, Kaku and his 4 sword style.
Whoever wins amongst them is the WSS.