AFAICT, a standard Linux rootfs is read and/or executable by any regular users and read/write and/or executable by root. If you login as a regular user, you can not write to a rootfs. Please list the root directory of /usr/bin, i.e. ls -la /usr/bin/. and post it here to show what the directory mode is.
/usr/bin was never writable, because it is stored in the read-only squashfs.
What exactly does you addon do and why does it need to write into this path?