Because if you disable caching in this manner it will be shown as low in the options menu, saving and applying any graphic setting at anytime after that will re-enable precaching but keep the low 0 setting. I did try enabled 0 quality 3 which would at least set it ultra but I’m not sure it worked so well.
And no not really, if (as they do occasionally) Asobo changed the format of the Cfg your backup would probably cause all sorts of problems. Setting read only could have the same effect although I think maybe the devs are now wise to that and will delete and rebuild your Cfg regardless.