I totally doubt that Canon makes lenses with soft edges on purpose. I think the soft edges are a result of technology limitations and cost saving.
In the case of the 50L (both of them), it was an intentional design decision by Canon.
I'm pretty sure that you can introduce spherical aberration through plugins or other software components if you really so desire. What you can't do is correct for poor image quality at capture time.
There are polarization effect filters for post-processing, but they cannot properly replicate the effects of having a CPL on your lens at capture. Similarly, adding spherical aberration in post will not correct for poor bokeh in the captured image.
Anyway, in the main the comments above about justifying Canon's current design and product are more about trying to ensure that people who worship Canon find a way to present Canon's offering as good and justified so that they feel good about owning Canon products. That's it. I'm sure someone will argue here that this comment is wrong...
Comments like the above are mainly about bashing Canon, made by people who have an inadequate grasp of the concepts behind lens design (and in one case, the inability to distinguish a lens from a camera).
...but you don't see anyone saying that they wish the 70-200/2.8 II had soft focus like the 50/1.2L and so on.
Not in those words, no. But plenty of people have said that they prefer the bokeh of the MkI versions of the 24-70/2.8 and 70-200/2.8 IS, or at least acknowledged the less-that-stellar bokeh of the MkII versions. When push comes to shove, they may not want to trade sharpness for better bokeh, but I suspect many people aren't even aware of the trade-off.