I have managed 40+ years bike touring without using waterproof panniers. Granted, few were available in my earlier years but I have not found myself at a disadvantage by merely using rain covers and dry bag stuffers (more recently). I have survived extended torrential rains in Alaska, Canada, US, Central America and South America including the Divide Ride even while only using down sleeping bags and jackets.

I have to admit that I would not and cannot float my bike across a river using waterproof panniers as depicted in some journals or manufacturer websites.