> While if you are living somewhere, it's really important to know a lot of basic supermarket items that you wouldn't prioritize for other usages.

Why? Having spent a good amount of time living in Shanghai, I found it important to be able to understand menus. But there's no pressure to know the words for supermarket items; you can just go to the supermarket and look for the item.

Otherwise your point is correct; all semantic words are equally difficult and which ones you know depends on the things you like to talk about. Grammatical words are more difficult, and more important, but this is so widely understood that language-learning material already treats them as an entirely separate class of things to learn.

> Also, while it's easy to calculate word frequencies for stuff like newspaper articles, there aren't any good statistics (last I checked) around just normal everyday conversation. Because that stuff isn't getting recorded and transcribed.

(1) You seem to want COCA, which includes a bunch of transcribed telephone calls.

(2) Word frequencies are still the wrong concept. If you want to understand a particular document, you need to understand almost all of the words that appear in that document. (You'll be able to learn some of them from their use in the document.) If you decide to learn a list of "frequent" words, you're unlikely to be able to understand more than a couple of isolated sentences in any given document.

> you need to understand almost all of the words that appear in that document

I didn't learn English by reading vocabulary lists or dictionaries. Pretty much all was learned by context.