-- more control of depth of field / shallower DoF ability?
-- faster shutter speeds?
-- less chromatic aberrations?
It doesn't intuitively feel like sharpness should be a factor -- even cheap kit lenses usually get that right.
The chromatic aberration is an important but subtle effect. Remember that lenses are multiple pieces of glass, and every interface diffracts the wavelengths of light like a prism. One of the considerations in lens design is converging all those different wavelengths of light in the same place. Not just at one point, but at every point across the image plane.
Poor lenses might do the well in an area. Good lenses do it everywhere.
The static aperture also helps tremendously of course, yes - nice bokeh with a tight zoom means you can easily get candid portraits that look great from anywhere in the room.
L glass is also a very interesting used market - those things basically don't lose value IME.
Other than the actual business side of things, pesky details like getting clients. And the massive stress of shooting a wedding. I was happy to do it gratis for family, but I don’t think I’d want to deal with paying clients.