There were a LOT of things the Allies chose not do (consider the Evians Conference, for instance) well before hostilities broke out.
I don't disagree that once fighting broke out, there was nothing to be done. But there was plenty to be done way back in 1933 when the first concentration camp was opened.
I'm also not denying that after the Wannsee conference, the efficiency of the Holocaust really picked up [1]. I'm saying that it's a mistake to believe that anybody in control was at all on the fence about what to do with all these undesirables, until they held a conference and said "why don't we just kill everybody?". They'd been trying for years, and it turns out that just shooting 17 million people is a lot harder than it sounds.
0. https://en.m.wikipedia.org/wiki/Timeline_of_the_Holocaust
1. https://theconversation.com/quantifying-the-holocaust-measur...