“Because we have two thousand years of Mesora behind us & MO only has fifty”
The mesorah of over 2,000 years ago also recognized the permissibilty of men having concubines, more than one wife, taking non-Jewish women as P.O.W. for possibly not-so-kosher purposes, the mitzvah of yibum (which is now almost exclusively modified to chalitzah alone, today), involuntary servitude, etc., etc., etc. Not everything that is Mesorah is practiced today, for a variety of sound reasons. In a time and place when the strong possibility existed of milk coming from a non-kosher animal, such as a mare or pig, and being sold as regular milk, it was absolutely essential to ensure that only the milk overssen by a Jew would be served as milk. We live in a country where all milk is government inspected and graded, and in order to receive the gov’t OK, it MUST and can ONLY be cow milk. Hence R’ Moshe gave the heter. BTW, even if the heter was “in case you cannot get Cholov Yisroel,” nevertheless if it is kosher, it is kosher. It cannot be a little bit kosher, because if it only is a little kosher, it means it is NOT kosher. A rov cannot give a heter on on issur, except to save a life. I think we can agree it is not life-threatening to refrain from drinking milk. A little inconvenient, possibly, but plenty of people are allergic to milk and all dairy, and do just fine.