Depth of field does scale perfectly with crop factor, and that's a big thing. f-number is a ratio of apature to length. A 400mm f/5.6 will have a 71mm apature.

Depth of field does scale perfectly with crop factor, and that's a big thing. f-number is a ratio of apature to length. A 400mm f/5.6 will have a 71mm apature. An iPhone lens does not (despite having a better f-number). The light is spread out more, so you'll a higher ISO to take fast photos, but let's say ISO is proportional to sensor size. A larger sensor captures more photons, so it can have a higher ISO. That is a law of nature. As you say, ISO is neither deterministic nor linear, but I'm more confortable considering ISO to scale with crop size than pretending it's constant. If we are ignoring ISO altogether that's fine, but then there's no point talking about the amount of light hitting the sensor. That leaves diffraction, but I don't really understand diffraction.