The recommended 72 hours of a survival kit is meant to serve as a buffer. This buffer is an amount of time that you have needed resources readily available, such as water, shelter, food, and fire, for enough time to construct or acquire additional resources for additional time.

For example, you don't just carry 72 hours of water and stop at that. You also build a fire to melt snow, carry water purification supplies, use stills, transpiration bags, sponges, etc. to gather additional water.

I agree that you cannot carry everything and cannot prepare for everything. That's why we all visit and read Doug's work here to find out what equipment is the most essential, effective, and versatile to have for maximum efficiency.