It is not worthless at all.

It is.

Studying cultural changes through the lens of evolution is very useful and enlightening.

Agree, but different topic and not dependent on the definition of a meme.

I would argue your narrow definition of "meme" is worthless because we already have a term for what you are describing - they are called "image macros".

But I never narrowed it down that far, I only excluded (explicitly and so far) screenshots of texts. You misinterpreted that explicit exclusion as me implicitly narrowing the definition down to only images.
Now, granted, this medium narrows down what can be shown to basically images (videos are images, too) with text and/or sound, and image macros are a very common meme format, but out of this medium there are other forms of memes, too (for example deliberate moves of ones body like the Dab, Tik Tok dances or a mic drop, or dropping a side reference in a conversation). Even within this quite limited medium there are image based memes that do not need any text (like the seemingly infinite variations of the loss meme). All these forms (and examples) have one thing in common: they take an existing idea, symbol, practice manneurism, etc and recontextualize it, creating intertextual references. A screenshot of a text with nothing by its side does not do that. You can make a meme out of some of them (like posting a sign somewhere that redraws the lines of some rules just so much that nobody notices), but the screenshots of stories or jokes are not memes by themselves. And the letter A all by itself isn't either.