I have a friend who had a mare sever her extensor tendon. She was found upside-down in a storm drain, mud and filthy water all over her poor cut up leg (which was still tangled in the fence she caught it in), and the vet said that IF she survived, which was "doubtful due to the high risk of infection", she would never be fully sound, would never jump again and would only ever be sound for light work if any.
Fast forward a year and a half, this mare is back in work, jumping, packing around a young girl (she's leased) like nothing ever happened. There's barely even a scar, either. The extensor didn't heal (and wasn't surgeried because mare had been injured too long unbeknownst to owners - she injured herself sometime between afternoon checks and morning checks), but the other tendons took over that job, so she's effectively normal.
Vets don't like to say "oh yes horse will be fine" because if it's not, then the owners are devastated and usually peeved with the vet for giving them false hope. In all honesty it depends on the horse and its individual will to live.