I think you're looking at the box office out of context here, surely you can't be expecting a film like Tenet to bring in the same numbers as a comic book film or established known franchise.
The reality is Nolan's films have all been extremely successfully within their defined genre;
- Inception is one of the highest grossing Thrillers.
- Dunkirk I believe is the highest grossing War film
- Interstellar is one of the highest grossing Sci-fi films
What Nolan has achieved is far more impressive than him directing the next Marvel or Star wars film to gross billions at the box office (Captain Marvel director's proves my point).
I'm glad Warner Bros has the full support of Nolan and I would include Denis Villeneuve on that list too, the box office for Blade Runner was a criminal and even if Dune flops I still believe they will give DV one more shot at a big budget film.