It's true, I didn't consider this enough. But I had a way to create a key without ever possessing the original. If it could only be copied from the original, as even a semi-competent implementation of a magstripe would be, there would still be the chance that someone notices a theft of a key, and it's just harder to pull off if you have to find and covertly steal a key.
My point is that being able to trivially hijack arbitrary identities without even knowing the person let alone physically finding them, is not "good enough." It'd be like if you could make arbitrary car keys with just a VIN, and the VIN is displayed prominently, it would be silly to say "Well now, it keeps honest people honest, so it's good enough."