Assuming perfectly play from tanks/healers, yes, you would not die regardless of gearing. However mistakes will happen. Assuming a perfect tank and imperfect healer, the perfect tank will be more likely to die to boss melees going unmitigated due to sheer chance than to a broadcasted and known spike mechanic. Assuming the imperfect tank and perfect healer, the tank will be more likely to die to broadcasted and known spike mechanics due to being larger, quicker spikes in damage that cannot be healed unless mitigated. The point of gearing/play as a tank is to assume the imperfect healer who may not always get off that clutch heal when you take a spike in damage, while the point of gearing/play as a healer is to assume the imperfect tank who is more likely to take those unpredictable spikes in damage.