On the other hand, the word "this" is pronounced as /ðɪs/ in standard Englishes, with /s/. And its etymology suggests that it was also /θis/ with /s/ in Old ENglish. It is constant, its pronunciation did not change.
So why did the /s/ change to /z/ in "is"? Or why didn't the /s/ change to /z/ in "this"?
(I just noticed that the Old English pronunciation of "this" has /θ/ but now it has /ð/ but I am not asking about that. Ignoring that would be better. My main question is about the ending "s").