Used to bother me too, as a mathematician. Then I learned about linguistics. If enough people use words a certain way, then it's vernacular. Sadly, I think this usage is long accepted.
It cost 10X less if we use 'Y' over 'Z'.
My brain always short circuits and says "I think they mean it costs 1/10th...."
"There was a 5 times decease in crime..." uggh, just give me the damn numbers. Is crime now at 1/20th the previous level???
I know they are telling us it's less, and I don't even care about the story; but i waste brain power trying to grok what they really mean.
Certainly there are more important things to fret about, like: "Tim went to the store with he and I.' Arrgghh!!!
I try to say "{x} is ten times {y}", which is, as far as I can tell is unambiguous. (Mostly, what's ten times colder?)
To me, "x times more than y" means the same as "x times as much as y" only "y×x" and "x times less than y" means "y÷x". I guess this might seem ambiguous if you bracket the expression like "x times (more than) y", but in practice it's bracketed like "x (times more than) y".
(Native American English speaker)
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I also wouldn't say that 6 is "75% less than 8" if your thought is that "more" just indicates a bigger value.
Languages change and are influenced by their use, so you need to pick your battles. If something can be unambiguously understood then you're going to be fighting an uphill battle and it's only a matter of time