I’m not a RT dev, so take this with a grain of salt. The semantics of the word “output” imply a rendition that will be used for display somewhere. ProPhotoRGB is definitely not a profile for that purpose. Export for downstream software is not the predominant use of Output…
Some folk might want to use another profile; ProPhotoRGB is challengingly large. Me, I’d use Rec2020, no good reason… 