When you brought these arguments up on Lobsters <https://lobste.rs/c/dsm1up> I said it wasn’t cut and dried but that my general feeling was of disagreeing with you. I’ve thought about it more since then, and firmed my position up: you should make sure that the presence of initial-scale=1 won’t affect anything (we agree on this point), but that if you make a mistake in this area, it’s much better to have it than not to (we disagree on this part). Of your five points:

Your first point is correct. It could be reasonable to omit initial-scale=1 during development, or have something that complains on width overflow.

Your second is unavoidable: omitting initial-scale introduces a worse failure (excessively small text).

Your third I disagree with: when overflow occurs it will almost always be obvious that you could scroll, because you’ll have text or image that runs to the inline-end edge of the viewport rather than being contained by a page margin as is universally practised.

Your fourth isn’t useful—by that argument all documents should be zoomed out until the entire thing fits on screen; but we don’t do that in general because having to manage the zoom to get a useful result is unpleasant, yet that’s what you’re advocating for.

Your fifth is irrelevant, I think entirely unrelated to initial-scale.