I would also bring up that it is a best practice to store backups of your seed phrase in multiple mediums of storage. So if you do decide to store one copy that is written on paper, you should store another copy of your seed in some medium of storage, such as an encrypted USB stick or encrypted HDD. This way if your ink does fade, you will hopefully still have access to your other backups.
I understand the reasoning behind this, and if that makes you feel safer and/or fits with your threat model, then by all means go ahead. But again, as I discussed above regarding steel back ups, I find this unnecessary.
Lets say that I have three copies of a seed phrase, all hand written on paper (using good quality ink and good quality paper), stored in three separate physical locations. I know that under the reasonable conditions of temperature, humidity, etc., that the copy stored on site in my house will experience, I'm going to get decades out of it before the ink fades to any significant degree. In terms of my offsite back ups, I'm still checking on them at least every few months so I know if they have been compromised or damaged in any way. Even assuming these offsite backups are exposed to such environmental extremes that the ink fades in only a year, I'll find out before they become unreadable and will be able to replace then as needed.
If you have created your seed phrase on an airgapped device then there is little additional risk to creating a back up of your seed phrase on an encrypted USB drive. But very few people do this. If you have, on the other hand, created your seed phrase on a hardware wallet, which thousands if not millions more people do, then creating an encrypted USB drive back up poses a significant additional risk since most people do not have a properly airgapped device with which to do so.
You make good points, but I think your points are outside of the scope of the OP's question/problem. If you are comparing storing a backup using medium A and medium B, in order to compare the two mediums, you need to assume the same security measures are taken, unless doing so would not be possible.
You do not necessarily need to use a USB stick for one of your backups, but the medium of storage for at least one of your backups should be different than your other backups. Every medium of storage has the potential for data loss under certain circumstances, and the only way to mitigate this risk is to use multiple storage mediums.
I would also point out that not all seeds are associated with cold storage. It is generally a good practice to use a seed for your hot wallet, and your hot wallet should also be backed up, including using multiple storage mediums.
I would also bring up that it is a best practice to store backups of your seed phrase in multiple mediums of storage. So if you do decide to store one copy that is written on paper, you should store another copy of your seed in some medium of storage, such as an encrypted USB stick or encrypted HDD. This way if your ink does fade, you will hopefully still have access to your other backups.
I have several USB devices that suddenly died on me, so I am not considering them a viable option for any important backup.
They can also create errors with operating system if they are encrypted resulting in easy disc format and permanent data destruction.
I wish there was some physical switch that disables writing like it was with old magnetic tapes long time ago.
Yes, there is x problem with y medium of storage. That is why you use multiple mediums of storage so you can reduce the overall risk of losing your seed.