Why w is so unstable?

Oui, oui, les Gallois volent tout les lettres W en Français.

(Note: the chances of me getting the grammar right in that sentence are near zero. No doubt someone will be along shortly to correct the errors.)

Missed the edit window, but I’ll correct one error myself. It should be “la langue française”, not “en Français”.

Sound changes in language almost never have anything to do with spelling. As noted, for most of history, most people were illiterate. Besides, the OP mentioned Greek, Sanskrit, Hebrew, Chinese and Japanese, none of which even use the Roman alphabet and couldn’t have been affected by the letters used in spelling English or any other language that uses that alphabet.

–Mark

BTW, I’m referring to systematic changes that affect all words containing a particular phoneme, as the OP described. I’m aware that there are “spelling pronunciations”, where the pronunciation of a single word changes because of its spelling, like “calm” pronounced with an L sound, “sneeze” with an S (previously “fneeze”), “buoy” as two syllables, and the British pronunciation of “solder” with an L. Many of these are recent, reflecting more widespread literacy.