Any metal object can crack under the wrong circumstances. Magnafluxing each slide to check for fracture lines would be fairly expensive but this seems to be the approach SIG took with at least that one particular production run of slides. You have to give them credit for that.
This could just as easily happen to a Wilson Combat handgun, a Glock, an H&K... you name it. The slide doesn't have to be made of pig metal; it can be forged or CNC cut from cold stock. And it's not limited to just firearms; it's seen in things like engine blocks, brake rotors, bolts, nuts, etc. quite frequently.
As long as the manufacturer stands behind the replacement or repair of the firearm and no one gets hurt... I see it as being mildly distressing but ultimately a non-issue. Guns are mechanical. Mechanical things can and will fail.