Only if they made their "backups" writable as well

It's difficult to create backups if you can't write into the backups storage.

The way I do it is I create backups locally and then copy them to my NAS over ftp(with a password), instead of samba. Hopefully, that way any ransomeware would not be able to encrypt my nas as well, as it does not have any publicly accessible folders on the network.

The problem is that ransomware doesn't have to activate immediately, so you can end up with multiple copies throughout your backups before it takes effect. Hence, restoring from backups may not solve an infection.

write != overwrite

So changes to files don't get backed up?

Obviously you back up changes, but not to the same place as the old file. Backups should be append only. So that if you at some point you realize you've started to back up encrypted data you can just roll back to the point where the data wasn't encrypted.