Could work. How do you know what the least-frequently used key is if you can't store them, though? Would need some heuristics. Maybe it could write the first five keys it sees after power on on every power on, or some other useful heuristic.

Like, I do take your point but it does seem quite involved for the chance that it'll get them something useful, and they still need to gain physical access to the intact device, and trust that it never gets out or the chipmaker's reputation is instantly trash and potentially bankrupt. And we know from Snowden documents that, at least in ~2013 (when aes extensions weren't new, afaik), they couldn't decrypt certain ciphers which is sorta conspicuous if we have these suspicions. It's a legit concern or thing to consider, but perhaps not for the average use-case

edit: nvm it was proposed in 2008, so that it didn't show up yet in ~2013 publications is not too surprising. Might still be a general point about that 'they' haven't (or hadn't) infiltrated most cpus in general