Any metal part, subject to a load, had to be designed with fatigue in mind. Rotor will have parameters such as force and load cycles (over a range of forces,
Miner's law if your interested) and then introduce a margin for tolerances. All companies do this, it's the reason your washing machine fails the day after the warranty ends for example.
So your crank will have been designed for a given life and either through an unlucky stack up in tolerances (though unlikely give this is the second time) or the unique way you load the cranks has taken them beyond this life which has lead to failure. It is clear you have found the limit of the design but that doesn't mean it is an objectively bad design or indeed a dangerous product for the average user, but rather it may be unsuitable for you and why you would be in titled to think it is a bad design.
FWIW I am a masters student in material science and would be happy to discuss the mathematics of fatigue in more detail, but I don't think a public internet forum would tolerate this too well.