Writing down a seed still requires some kind of computer-like device to generate the seed. If the seed was generated on a HW wallet, then perhaps storing a seed on a USB drive would require an additional computer. However, if you remove that assumption, using a USB drive requires no additional security measures above using a paper backup.
The only time you need to no additional equipment to create an encrypted USB is if you are using a software wallet on a computer, and even then there is additional risk in taking a seed phrase out of a password protected wallet file and saving it in plain text in a text file. These risks are mitigated if you are using a properly and permanently airgapped system, but we both know that very few people do that and do it properly. If you are using a mobile wallet or a hardware wallet, then inputting your seed to a computer to create an encrypted USB back up is significantly riskier than writing down your seed phrase on paper.
If you compare the potential security risks associated with creating (and using) a paper wallet, and a wallet stored on an encrypted hard drive or USB drive, using a paper wallet would have all the security vulnerabilities associated with a seed stored on an encrypted hard drive or USB drive, and would also have additional security vulnerabilities.
I am comparing a properly created paper wallet (airgapped device, dumb printer, etc.) and a hot software wallet. Properly used then the paper wallet is exponentially more secure, but we both know that many people do not use them properly.
I think it is disingenuous to say "Well, if you do it properly then printing your seed phrase is fine", when we know that the vast majority of people will not or can not "do it properly". Far harder to make a critical error when writing down your seed phrase than when printing it out.
The variable that increases the benefit in your comparison is the distance between backups, not using a single medium of storage.
My point exactly. The important point of your 3-2-1 rule is at least 1 back up stored off-site (although I would argue for more than this if you can safely do so).