It bugs me when people say “the thing is is that” (if you listen for it, you’ll start hearing it… or maybe that’s something that people only do in my area.) (“What the thing is is that…” is fine. But “the thing is is that…” bugs me.)
Also, “just because <blank> doesn’t mean <blank>.” That sentence structure invites one to take “just because <blank>” as a noun phrase which my brain really doesn’t want to do. Just doesn’t seem right. But that sentence structure is very common.
And I’m not saying there’s anything objectively wrong with either of these. Language is weird and complex and beautiful. It’s just fascinating that some commonly-used linguistic constructions just hit some people wrong sometimes.
Edit: I thought of another one. “As best as I can.” “The best I can” is fine, “as well as I can” is good, and “as best I can” is even fine. But “as best as” hurts.
English intensifiers tend not to follow Boolean logic flows very well (think of double negatives). Instead, try to think of it as a little bit of extra data for your or the speaker’s benefit. “It’s all but impossible” does mean it’s possible, as you say, but there is more there. It means, “while this is possible, it’s so difficult or unlikely that we cannot count on normal levels of luck or effort to help us; you should reset your expectations accordingly.”
Your other example is similar. “It’s all but complete” tells you that the project or event is almost but notyet complete, but more than that. It means “This is very nearly complete. It is so close to complete, in fact, that the remaining time will be trivial. I suspect or know that you are eager for it to be complete, so unless doing so is all but impossible (😁), please try to be patient just a little bit longer.”
“All but” is a way to linguistically make a fine gradation in levels of “almost”.