On a bicycle with cantilever brakes and a separate straddle cable (not a "link wire"). If the main cable coming from the lever were to break/snap/fail then the spring tension on the cantilever arms would cause them to open up, thus bringing the straddle cable down into the tire.

On a knobby tire, especially a burly MTB knobby tire, this would cause a sudden and dramatic decrease in speed.

Yes, Cachehiker-- you will feel the cable let go. When the main cable fails you will "feel" it as it brings the front wheel to a sudden stop.

That said, I rode a mountain bike for years in the 1980's and never suffered an accident as a result of brake cable failure.