If your root filesystem is readonly then you will indeed be unable to do application or file updates.
I’m not sure what you mean that ‘Memory A/B and Partial Update makes rootfs to be “readonly”’. That should not be the case. Doing an A/B update should not change your rootfs to be readonly.
Now, if you are referring to robust binary delta updates, then indeed it is required that the root filesystem be readonly. This is to ensure that the contents of the filesystem have not changed. If it has changed, then you will not be able to apply the binary delta. This does imply that you cannot use application or file updates in a system that also uses binary delta updates.
Drew