I think this perfectly encapsulates the problem. The inescapable conclusion - regardless of directorial good intentions - is that whatever was meant originally, the end product was designed and delivered in colour. Therefore it should stay in colour.

Personally I think it is a shame there aren't more monochrome films for appropriate subjects. Yes, big-time directors can be choosy about it - Spielberg with Schindler's List, Burton with Ed Wood, and more recently Fincher with Mank - but on the whole, as you say, monochrome remains a mainstay of the arthouse brigade (Ida and Cold War being recent cases in point - both brilliant films, mind you).