It doesn't even have to be waterproof to prevent 99% water damage cases. It just needs to be water resistant against liquid pours from the top, like how a car can withstand rain.
There are backpacks with bottle holders on the outside, but in my experience, those aren’t very good at holding a bottle.
Not bringing a bottle probably is the best option, but that may not be a good option in very hot climates.
And some backpacks actually have working bottle holders on the outside ...
How would you avoid water damage due to the scenario? Or do you consider it normal and unavoidable?
Most of my cases where me toppling over some classes or bottles in a way where the liquid ended up directly on the open laptop.
On the other hand in my bag I normally have the Laptop in an additional sleeve, sometimes a double one, so any case where liquid ended up spilling in my backpack it at most lead to a slightly damp laptop due to the sleeves getting wet but nothing which causes water damage.
As a side note if used correctly a simple plastic bag (like a thing plastic shopping bag) as an additional sleeve around you laptop sleeve can be a trivial cheap way to get a surprising amount of waterproofing a laptop in a backpack. I don't do this on a daily basis but when traveling or similar I always hope to not forget to do it.