Van Gogh lived in the XIX century. How many centuries must pass before we stop calling that modern? What words should we use for things which are actually modern?
Modern art is a technical term in art history. Maybe 20th century art? But even then I'll bet people can find something made in the 20th century they like. Maybe they'll have to give us more info about what they mean by "modern art".
I know that. I’m arguing the term is dumb, since it waters down the natural meaning of words, making it harder to communicate effectively. Art historians should come up with a different, more specific term.
Art historians are the ones who invented the term. Arguably it's everyone else who has tried to redefine it into something else.
Art historians invented the word “modern”?
Contemporary.
If anything, art history defines "modern" later than the real historical meaning ("modern art" starts in the 1860s after the invention of photography). In history modernity starts in the 1500s - in Western culture history is divided into ancient (dawn of civilization to fall of Rome), medieval (fall of Rome to the Renaissance) and modern (Renaissance to now). You'd need a fall of civilization or something to justify a new era.