opsec 101 - don't use other people's devices / machines

Do you never, ever, use devices not in your full possession?

Isn't that what you do?

If you have deals with many strangers do you not expect to get your hands dirty in some way?

I definitely don't store my SSH keys on stranger computers. I however used passwords on my relatives or customers devices on numerous occasions. Don't pretend you never did, I'm not going to believe you.

Sure, I've used other machines in the past. I prefer not to though in most circumstances and assume that any account / password that I use on said machine is compromised from then on.

This was something I changed when I realized that a tax related government agency's machine I was using about 10 years ago man in the middled SSL connections to my online banking. Who knows what was logged and what the retention on that was? I don't have time or the inclination to ensure every relative's / customer's devices are secure, so I use my own or go without.

That should be soluble though. E.g. standardized smartcards that cellphones and terminals know how to communicate with.

In theory yes, although with caveats. You'd need to match the degree of anti-tamper measures currently present in POS terminals to all consumer devices.

In practice, it would mean replacing most of the world's user-facing computer infrastructure.

You can get computers with smartcard readers from virtually every major manufacturer (except Apple, because Apple). The most frequent use case is government users who need to support the DOD Common Access Card (CAC) standard.

The card reader doesn't have to be any more tamper-resistant than the rest of the computer. On a public terminal, it should probably be built like an ATM's card slot (and the rest of the machine should be similarly armored), but on a personal computer it doesn't need to be any more robust than the keyboard a user would otherwise type passwords in with. The card-computer communication is all encrypted anyway, so even if it's built like any old USB peripheral it's still better than typing into a keyboard.