Bikepacking - a viable alternative to racks & panniers

Why are panniers necessary if I go over the Alpes? Also which saddle bags specifically do people advise? I am probably going to try and avoid going through the Alpes if at all possible though.

How much in litres are you going to be packing and how much are you willing to spend on luggage?

As for bib shorts... Some people even ride in clothes without padding! Crazy huh?