Oda is either holding Gorosei massively back or they aren't IMU's top fighters
Since we get to know about Garling and the god knights and since it was speculated Shanks' got a twin I got more and more convinced that atleast one guy besides Garling will trump the Gorosei.
Thematically it also makes a lot of sense. If Oda goes for two antagonistic Shanks copies, one in his prime- one not, he basically has two additional haki god-prototypes he can scatter across the EoS battlefield, which in turn does very well for the final war tension and match ups.
Without them that haki god- prototype would go down with Shanks, who likely gets clapped by BB. Luffy relies too much on his DF shenanigans and Mihawk & Zoro are too deep into their WSS thing, it's why none of them authentically could and would get used as a haki god- proxy.
Now you have basically 2 additional Shankses/Rogers/Rayleighs to use, that can be put up against awakened logias or some CoC bolstered dragon claw style.
And by default I don't think that that haki god-prototype, even if not in his prime (Garling), would have lower value than some glorified Rusukaina beasts.