We will have to redefine Wikipedia?

All what you wrote does not contradict the current rules of wikipedia. In addition to w:Truth, I would recommend us to review w:Wikipedia:Verifiability, not truth, which I believe addresses most issues of the thread. For example,

Relativism of knowledge in sciences (i.e., changing of the "absolute truth" of science with time) is covered by historical articles, which explain what "truth" was 500 years ago, in articles, such as w:Alchemy, w:Aether, w:Flat Earth. We have articles about "not-so-absolute-truth" (such as fringe science and fraud), e.g., Time Cube.

Relativism in Arts is covered by recognition in wikipedia that an opinion of a person of repute in particular area is a fact of note an encyclopedia. Yes, corpulent women of Rubens will fail to be w:supermodels today, but an encyclopedia may explain how the concept of 'beau femme' changed in time and around the world.

And so on. Therefore I don't quite understand why we have to redefine wikipedia, per original poster and what opinion s/he argues with.