@takata - You're clearly right that if the aircraft can maintain wings level at a suitable attitude, it can glide for a long time from FL350.
What if the aircraft entered a storm cell? The huge updraft/downdraft velocities inside the storm system could make the level aircraft lose altitude very rapidly?
I only fly gliders, not large aircraft, but I've flown in pretty rough conditions where we were wings level at normal attitude experiencing 10kt downdrafts (which is a very big downdraft for a glider at little more than 1000ft!). Under those conditions you can lose altitude very rapidly even if the aircraft is in a normal, non-upset condition.