Why canvas, not coating
The trade-off nobody announces
Coated fabric is perfectly waterproof as it leaves the factory. The coating is a layer of polyurethane applied on the inside; it does not wear away from the outside, it hydrolyses slowly, on its own, whether the bag is used or not.
After three to five years it lifts off in sheets. At that point the bag cannot be repaired: the lining peels under your fingers and there is no way to re-coat a fabric in place.
What we chose
An uncoated heather canvas, tighter in the weave, treated with a surface water repellent. It takes on water in sustained rain, and that is the price.
In exchange it ages without collapsing: the repellent wears off and is reapplied from a spray can, the canvas is repaired with thread, and a tear does not spread.
What it costs you
A bag that is not a waterproof bag. For a laptop in the rain you need a sleeve, and we would rather say so than let you believe otherwise.
We did the arithmetic differently: something you keep for ten years that needs a sleeve twice a year seems better to us than something flawless for three years and useless after that.