"The benefit to uSD is that it is backwards compatible to SPI flash by just hooking up to the right pins and clocking it accordingly."
But SPI sucks and that's not the "benefit" of uSD AT ALL. UFS is built on an industry standard, just a newer, better one that isn't SPI.
"If I'm not going to do that, then what benefit does this standard offer over an internal Type-C port, the next iteration of uSD, mini/micro-SATA, NG.4(??), or CompactFlash?"
It is removal storage that is much smaller than SATA, USB, and CF devices. Duh. Next iteration of uSD? What is that? It is probably not better than imaginary future products that you dream up.
"Seriously. CF gives you IDE support, uSD gives you SPI support, NG.4 gives you PCIe x4 support, and USB Type-C gives you up to 10 megabit USB support. Any price, space, and performance level is already covered. And most of those standards already have converters between them."
Of those, only uSD offers the form factor that this standard targets and it is stuck with low speeds and a terrible protocol that has no error correction or detection. It appears ultimately that your complaint is that they didn't use some PCIe derivative instead. Why should you care?