> it takes care of my grammar blindspots (damn you commas and a/an/the articles!)

There are plenty of pre-LLM tools that can fix grammar issues.

> Can you please share what and how gets degraded?

I'm not the person you asked, but IMO LLMs suck the style and voice out of the written word. It is the verbal equivalent of photos that show you an average of what people look like, see for example:

https://www.artfido.com/this-is-what-the-average-person-look...

As definitionally average the results are not bad but they are also entirely unremarkable, bland, milquetoast. Whether or not this result is a degradation will vary, of course, as some people write a lot worse than bland.