It's mature enough that it's known there cannot be a "robust" system like you seem to be hoping for.
At some point down the chain, you trust someone else with your computer's off switch. If they give it away/sell it/have it stolen, a stronger system merely means you're more assuredly screwed because there's less they can do to prevent it from working as advertised.
And this is all aside from cryptographic weaknesses. Sure, there are strong / robust systems, but at some point the ultimate authority lies somewhere, somehow, and it can be taken. Even something like BitCoin, a nigh-authority-free system, is vulnerable to this; if enough malicious computers perform enough malicious calculations, they can convince everyone that their transaction history is the correct history. Or a virus could do their work for them.
1) you have to assume that the implementation is correct so that only somebody with the key can preform the remote kill, and ensure that the attacker can't reset the key. If you get this even slightly wrong, somebody will figure it out. How confident are you that this will be implemented without error? Would you bet all your companies hardware on it?
2) If the attacker gets you're key, you're screwed. And not just "somebody is signing shit with your key without your approval, time to revoke the key" kind of screwed, but rather "time to toss all your hardware into the dumpster" kind of screwed.
If they can kill it, others can too. Be they script kiddies, bored geniuses, or businesses or governments that would like to see your computer die.
The trick is that if there is such a desirable off button, it will be discovered, reverse engineered, or leaked. There's no "if", only "when".