The only bit we're lacking is the "tied to one's bank account". The rest already exists in the form of yubikeys and other hardware security tokens.

Your bank/credit/debit/etc. card is a “physical token with a crypto key that is protected by a password and tied to one's bank account”. FIDO and EMV even both use the same underlying ISO/IEC 7816 and 14443 protocols for communications.

Ah! I forgot about my debit card. I only use it ~once per month. Yeah, we'd need an additional piece of hardware to be able to connect it to our computers but that also could enable doing chip-based transactions over the internet so I think it would absolutely be worth it.

I know this was sarcasm, but bank card is not appropriate because you should have one hardware key for all services produced by an independent provider.

Why would I want to have one key for all them? To lose access or get them all compromised at the same time?

I think your threat model is bad — the isolation of accounts offers little and most people have effectively only the security of their email provider anyway.

That said, buying several keys would be a natural and happy path solution for the paranoid. Make sure to get three for each account (same as primary) as either you need a backup or the extra key offers no extra security.

Okay, I will make the "S" mark bigger next time.

Not how it works. One key can keep dozens of entries.