Shticky Guy who wrote: “After all, why would we still keep 2 days ??? ??? in ??? ???? when we no longer have a doubt when rosh chodesh is!!”
See the Mishna Brura on Yom Tov Shayni who says a totally different explanation for two days Yom Tov. Because of the leangth and bitterness of the looooooooooong golus, we will forget the cheshbonos and come ch”v to eat chometz on Pesach. (We may end up in a concentration camp and not have a calander with us to know the exact day!)