Light coming from the lens and forming a point (ideally) has the shape of a cone with its base at the lens.
That is why wide angles are so sensible against defocusing errors.
The greater the angle at the tip of that cone is, the faster and intenser defocusing leads to blurring.
Light coming from the "outer rim", which is only revealed at open apertures, forms a cone with a greater angler than light coming from the central parts, which are always used, even at high F-Stops=small apertures.
The light forming a point comes from the central beam, beams from the outer rim and all between.
But: Due to the different angles, the light from the "outer rim", which is only there if the aperture is wide open,
is more sensible to the blurring effects of defocusing.
The light from the center is not.
Therefore - the larger the amount of light from the outer rim is, the faster defocusing effects become visible, and we see that as low DOF at open apertures.
Theoretically you could put a center filter with varying density (transparent to grey or grey to transparent) at the plane of the aperture and use this to manipulate the behavior of the bokeh. You could make the transition from sharp to blurred areas softer or harsher. I guess, the more light from the outer rim is there, the harsher the transition would be, the more from the center the softer.
ADDENDUM: Erm ... I just realized my error above ... Yup - the bokeh should be softer. But generally digital files seem to have a harder transition - Maybe its the thickness of the image plane film vs. sensor which comes into play ? Or is it the construction of the modern digital lenses ... ? Or is it "just" the generally smaller formats/sensor sizes?
Sorry for the mistake ... I'd really like to see a test and have some enlightenment from the tech nerds here ...
What remains for me is, that from experience there is a clear difference between the sharpness behavior of digital vs. film imaging and I thought (which I had to revise) the described problem could be an explanation. Now it appears the opposite is true and the light loss at sensor level is probably working against the otherwise harsh sharpness of digital systems. So - if I look at it from this side, this could actually be something good ...