A common idea was "any single system fault shouldn't make catastrophe by itself". Nowadays it's "any single fault shouldn't jeopardize the mission and any two faults should still allow for safe return of the crew". The redundancies are designed in accordance to this.
From this perspective Gagarin would die if both accelerating stage worked longer and retrorockets failed. Only one of this two systems malfunctioned - so, barely, he managed to return unscathed.
From this perspective Gagarin would die if both accelerating stage worked longer and retrorockets failed. Only one of this two systems malfunctioned - so, barely, he managed to return unscathed.