You seem to have a high degree of confidence that this thing would only activate during shaking that’s severe enough to bring down the building on top of you. But that’s a very difficult thing to fine-tune, especially since it would need to activate before the building starts collapsing.
Trust me, I personally know the folks who put together the ShakeAlert® earthquake early warning system in California and it’s difficult enough to strike the right balance on even alerting people that a strong quake might be coming without creating a bunch of nuisance warnings. Now just imagine if every time you sent out one of those alerts a bunch of folks got trapped in metal coffins. I can definitely imagine scenarios where a mild quake or even a bad sensor triggers one of these and then the person gets trapped inside without anyone knowing to even look for them because their building is undamaged.
