It doesn't bother me particularly and I must admit I barely noticed it (hence no original CW) but there is more to other people than I could ever understand; if I can take thirty seconds of my time to prevent someone seeing something that makes them uncomfortable (especially given it is not exactly expected in this context), why not do so?

It costs me so little and could benefit others so much, the same as most other good manners in our culture, like saying "please," and "thank you."

It sounds like you think the painting's age means that it no longer has power, or at least, not power enough to warrant a content-warning. I would beg to differ, and argue that the fact that it still has the power to shock is what makes it "world-renowned".