Real backpacks should come equipped with a replacement zipper. In a secret pocket you will remember about it when you will need to remember.

The pain you feel when the zipper of your beloved 20-years old backpack fails is overwhelming. It rips through your soul. And all I want to do in that moment is to remember that I have a secret pocket inside it which keeps my replacement zipper. The feeling of having it there ready for me to fix my brother in arms will also give me an extra 20 years of good life. Small things. Matter. So much.

I much prefer the "dry-bag" style of backpacks that don't use zips (maybe for a small front pocket). I've previously cycled with many a brand of "waterproof" rucksacks and they always seemed to let in water through the zip in a heavy downpour. With my current rucksack, the top rolls down and then buckles together to form an airtight seal and that is absolutely waterproof and the contents would survive being thrown into a river. It's also extremely robust as there's nothing to go wrong apart from the material itself becoming worn through (not happened after years of heavy use).