Rav Schachter claims that even if you don’t believe the research (where I heard he found 98% Treifos) then you should hold all milk is Assur because of the Shishim issue. And I think everyone would know it by now if Rav Elyashiv Assured all milk.
The principle is that Yotzei Min Ha’assur is Assur. I haven’t gone over this in a while and now that I think it over I may be confusing two things. I may be thinking that it doesn’t apply to something that is only Assur because of an Aseh. I have to look at this again. But either way, it’s very clear that we have more reasons to add Chumros to a Lav that is Mefurash Bikrah than one which isn’t. That’s an issue in Hanhagah, not Halachah. I would think that would be Pashut.