Let me put it simply: Whitebeard got as far as he did not due to his own power, but his family watching his back.
Whitebeard's crew measured 1600 pirates, plus a large number of New World pirates to accompany that. Without these numbers, lesser marines (and I'm not even talking "super lesser", but captains + rear admirals, etc) would have had more free reign. And as strong as you are, at the end of the day a legion can overwhelm you.
Shanks' crew is definitely weaker than Whitebeard's in regards to numbers, though strength is hard to gauge (the division commanders weren't dissapointing after all). What probably would be the deciding factor would be whether Shanks could pull in allies. I mean, imagine if the Pacifistas and Sentomaru had hit WB's crew directly instead of the New World pirates. The battle would not have taken long to devastate the numbers of the pirates until only the strong ones remained.
At the end of the day, without those numbers, even if Shanks was worth 2 whitebeards in strength alone, he would not have won.