Steve, you later state you “really understand this stuff”. This comment, however, seems to indicate otherwise. What you’ve essentially said above is, (to use video as an analogy):
“you’re not losing anything if you import VHS-quality video into a modern video editor because your editor is set to 4k, it will output a 4k file after processing.” …it will indeed output a 4k file… but that file will still only be VHS quality, but scaled to 4k resolution. Now, I’ll grant that the analogy breaks down as there is not nearly the [perceptible] difference in quality between 16 and 24 bit as there is between VHS and 4k video, but it’s still reasonable to want the 24 bit files from Dorico to start with.
I have personally encountered the difference between bit depths of audio files; in particular, one of my sample set environments is noticeably different if you load in 16 bits vs 24 bits (or even 20 bits).