The intersection of two blades cause a diffraction spoke ...
NO, it's the edge of the aperture opening which causes diffraction. Diffraction is an edge effect - light passing near the edge bends a bit. If the edge is straight the light all bends in the same direction so you get a directional diffraction spike on your image. Lenses with rounded aperture blades cause the diffraction to spread out evenly in all directions which causes a diffuse halo around point sources of light instead of a diffraction star.
Also, because diffraction is an edge effect, it becomes greater at small apertures since the edge / area ratio increases.
To the original question, there is nothing wrong having all lenses with 7 aperture blades. I think it is a good feature. Lenses with 6 or 8 aperture blades produce less interesting defocus blurs - the shapes are too regular and "square". Having an odd number if sides looks more natural and organic to me.
The very first F-mount Nikkors had 9 aperture blades, but they soon reduced to 6. The earlier rangefinder lenses had as many as 16 aperture blades. Rangefinder lenses are always stopped down to the shooting aperture, they don't move when the picture is taken so the aperture mechanism can afford to be heavy and well-built. However SLR lenses must have an aperture which opens and closes very quickly - wide open for TTL viewing and then stopping down instantly at the moment of exposure. Therefore the aperture blades must be very lightweight so they can move quickly. Maybe the early 9-blade apertures were too heavy with too much friction to operate quickly, or they were too complex and expensive. During the 1960s and 70s the number of blades changed to 7, possibly because it is a nicer shape. Lenses with very fast aperture, and telephotos had 9 aperture blades to give the opening a rounder shape, since the shape of the aperture is more noticeable in pictures taken with these lenses.
