I think you're overstimating the number of people that ignored Mad Max in favor of MGSV.
I think if you look at that we only have data for 1 day, which is probably 1/3 of steam accounts, and then look at the difference of the number of reviews and concurrent users between MGSV and Mad Max, I think it's easy to extrapolate that Mad Max sales are at >100k. Couple that with the very agressive pricing it's getting and wonderful word of mouth, I wouldn't be surprised if it sells around half of what MGSV sells, at least in the first couple of weeks.