Earlier quoted context omitted.
My backup drive has a physical switch. I used to be meticulous in setting it to read-only except while making a backup. But now I just leave it in read/write mode because I lost my enthusiasm. It's not plugged in when not in use though, so it's a bit safer from this sort of thing. How would you do automated backups with a physical switch? Are you proposing nobody does automated daily or more frequent backups?
I suggest rolling backups. Tapes are also a possibility, as they can be written in append-only mode. Just remember that ransomware will encrypt any drive you hook up to it, including your backup drives. A physical write-enable will prevent that, and it should be disabled anytime you are restoring from backup. I want a write-enable switch even for simple tasks like copying one drive to another, just so I don't mistake…
What I was alluding to but didn't make clear was that physical switches are easily defeated by human laziness or mistakes. Why have a physical switch on drives you're copying if you might accidentally switch the wrong one or eventually stop bothering? This already happens with warning popups in Windows when you try to run an untrusted program, for instance. People get trained to bypass the security because it's just a tedious obstacle.
You personally might be careful enough to forever set the switch correctly, but people who didn't even know their hard drive was years-un-patched and internet connected yet left it there to get hacked also wouldn't reliably set the switches every time either.
Regarding accidentally copying in the wrong direction, I think a safer way than a physical switch is to show some details of what will get deleted. Maybe previews of images, tiny snippets of text from the files, a graphical view of the space they occupy, etc. Make it more visceral, like throwing away an actual book where you can see how many pages it has and what the picture on the cover is.