Advantages of full frame?

[neuroanatomist]

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I have used the 100-400 and really don't dig the push-pull zoom. If they made that range in a normal zoom lens, I would be all over it. I have no real problem with the IQ, just the functionality.

I actually find it to be beneficial. Often, when shooting birds for example, you want to just zoom to 400mm and use it. (Yes, the 400mm f/5.6L prime is cheaper, lighter and smaller, but it lacks IS). In rapidly changing situation like an air show or outdoor sports, the speed with which you can adjust the focal length is an advantage over a rotating zoom ring, IMO.

I don't think there are
mathematical or physical limitations on small sensors, at least, not
ones that are relevant to our discussion.

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Not of the sensor, per se. But as I mentioned above, getting the equivalent DoF of an f1.2 lens on FF at &lt;100mm focal lengths on a crop body isn't physically possible (barring the release of an f/0.75 lens).