For your use case. Making, as you mentioned, a huge number of edge case decisions. So if innerText was standardized to your use case what would it be good for other than your specific project?
Pulling large chunks of plain text as a string to process for any low latency tasks constrains the size of text you can reasonably work with. JS strings are immutable as well, that's a lot of copying. So you can't be grabbing the whole document at once with innerText, but you still want it to handle line breaks. So innerText is useful for a text editor where the chunks are more than one line long but still fairly short.
Another issue is this whole handling of converting lists and tables to some kind of plain text markup. Is that a two translation? Are tabs going to turn into table elements if I have them in a string and set innerText? That seems pretty crazy, but not doing so would just make it very weirdly asymmetrical.
I liked the article, lot's of good info and stuff I didn't know. But I did come away thinking "Thank you Mozilla, at least someone is pushing for sanity". How much horribly slower do you want the DOM to be?