You are not disagreeing with me; you are just missing one step. When I milk a cow, that milk is Kosher because of Chazakah and Rov. When I am looking at a container full of milk mixed from separate cows and I know for a fact that more than 1.6% of those cows are Treifos, then if I am going to use Lach Belach the milk would all be Treif because Lach Belach Min Bemino needs Shishim. Since we know for a fact that more than 1/60 are Treifos we cannot apply our early use of Chazakah and Rov is no longer relevant.
The argument you are making is correct. You are just ignoring the Halachik reasoning behind it. We cannot assume the milk is all Kosher just because when we milked each cow individually we said it was. Now that I can’t point to the individual cow that this milk came from I can’t use its Rov and Chazakah anymore. Rov is no longer relevant and a whole herd does not have a Chezkas Chayim because Anan Sahadi that more than 1/60 of these animals are Treifos. That is where the Rosh comes in. We treat this Ta’aroves as a Yaveish Beyaveish Veachar Kach Lach Belach (because each container that the cows were milked into is a Yaveish Beyaveish Ta’aroves) which the Rosh quoting the Ra’avad says you only need Rov, which we still have.