Re: house of cards question
> I guess what happens to them is the same thing that happens to all those decade+ old laptops and desktops.
Yes, the ASIC hardware is ultra-specialised, so it can really only calculate sha256(sha256(message)) < D, where "message" is the concatenation of the previous block's header, a proposed new block and a "nonce" (effectively a random number, though they are scanned in sequence).
This kind of thing is no use for, eg, breaking passwords, so if Bitcoin dies, the hardware is effectively useless.
https://www.bitcoinmining.com/ has a nice block diagram explaining this. See the "What Is Proof of Work" section.