This is fantastic! I had greyscale on many years ago, but I had to turn turn colours back on for google maps... but this trick allows me to have my cake and eat it too!
Looking at the example images, I was actually shocked that the app was so low contrast in B&W to be unusable. At least apps from Apple/Google. I would have expected their usability teams to be all over it, while expecting smaller app devs to need a pass on this.