People, you forget that in real life, it will take you many seconds to realize that something is wrong. Just a few seconds brain processing, and you have 120+ knots. And if both captain and co-pilot airspeed indicators (unlikely) have failed, you have no idea of the actual speed. That could easily be the recipe for an overrun on a short runway (see AirBerlin in Dortmund).
Unreliable airspeed should be a minor problem for a proficient pilot. Definitely not "unsafe to fly" in my opinion. And apparently not in the opinion of Boeing either (no mention to reject for unreliable airspeed above 80).