EMBTC
Eminer / Bitcoin
crypto

I haven’t heard of EM detections of graphite traces. If someone has research on this, I’d be interested.

I've always hated the solutions where you slide letters in for the reason you describe. reusability is not worth the risk.
Use the punch tool method. Each word is 4 letters. It might be easier to snoop on someone writing the words with a pen. You are likely to write the full word, and the strokes will give away more information than a punch tool.
I'm not really asking what the downside is to having a backup, I'm asking why you would be better off with a paper copy. Even though you might lose the steel plate, I have a hard time coming up with scenarios where a steel plate would be lost, but a paper copy would not.

If you can see the seed with some kind of EM waves, would some technique not be available for paper as well?
You also have to ensure you're using an ink that is archival, less of an issue as the ink is not in contact with the sun.
You can also hide the steel plate under ground. With paper, it would degrade. It would also be last if you hid it in a shed, and there was a leak or similar.
Easier for someone to find a piece of paper and throw it out.
I don't say that steel is the end all be all, but I don't think paper is a better option.

Side channel attacks. Depending on the kind of steel-backup, this may vary. Some have little metal letters that you have to put into place. How difficult is it to calculate the seed from the *remaining* letters you threw out? "I don't know" is enough of a reason to not use that.
With stamping, someone listening in on the number of hammer hits may infer the number of letters and weaken the seed. Even the time between hammer blows. A word with repeating letters will have shorter hammer-intervals.
See-through techniques. As simple as a pencil swept across a piece of paper put on the metal plate, even if it's in an envelope. Or any kind of EM waves. Not only x-ray. Again, which EM wave length can be used to see through the container of the plate and make out letters good enough to calculate the seed phrase form it? "I don't know" is not good enough of an answer.
When writing down or displaying the seed phrase, no matter the medium, paper, metal, display of the HWW, make sure no cameras (phone, tv, hidden pin hole cams etc.) see it.

True - DOGE is not a success or failure because of EM. Had he not pumped it at all, it might be even lower down in value. He gave it value.