Yes, we are talking about CPL's if you'll note the bold red phrase in my previous post, a very general and untrue statement. And I don't agree with you at all that GND's are surpassed digitally. Just the opposite; I've had terrible problems trying to correct overexposure in post, problems which are mitigated when I filter in the field. I admit I'm talking empirically as a long-time photographer, and you're trying to explain light physics, which has no application to my personal experience.
We do agree than any improvement in DR would be a blessing, but I don't know that Canon will ever be able to match other sensor manufacturers.
No-one ever mentioned CPLs until you brought it up in the middle of a conversation about GNDs and uneven horizons.
You can't digitally filter something unless that information has been captured by the sensor in the first place. That's why you can apply digital GNDs and colour filters (since the RAW file contains luminance and colour data) but you can't apply a true digital polariser, IR or UV-blocking filter (since the RAW file doesn't contain polarity, IR or UV data).
The reason you've had trouble correcting overexposure in postprocessing is likely because you didn't capture that part of the scene's dynamic range in the first place. If it's not blown in RAW, you can pull it back.
There are many ways to capture this DR. Having a high-DR sensor helps greatly. You can also use multiple exposures, if the scene's lack of movement allows for it. I find both of these methods far preferable to physical GND filters, which I only use if I absolutely have to capture the scene in a single exposure (e.g. shooting from a Zodiac, a helicopter or handheld) and the DR of the camera is insufficient to capture the scene in a single shot. The ability to mask in any way I want, not just a straight line, is far too useful, since the transition zone is rarely an uninterrupted straight line. Also, putting an extra piece of plastic between the lens and the scene does no favours for contrast and flare, and, on UWAs, the light falloff in the corners becomes unacceptable.