You think we have a working system producing a substance that is only ever harmful?

If it's ever helpful, that's already guaranteed to produce a U-shaped curve. Just like I say in the quote you pull.

(It's theoretically possible that mortality won't go to 100% at zero cholesterol. But that's not the quote you pulled.)

If substance X produces Z and substance Y produces Z, then removing X still gives you Z. Or, removing a substance that is produced by the body does not necessarily leads to mortality.

I'm not saying that Y exists, just that the argument is either incomplete or the logic unsound.

Well, the body can utilize energy by either an aerobic or anaerobic pathway, but that doesn't mean we'd be fine if either of these pathways were disabled. We'd die without the aerobic pathway and have much reduced capabilities (probably fatal too, I'd guess - certainly if a tiger is chasing you) without the anaerobic pathway.

If a mechanism has been produced/preserved by natural selection, it's hard to believe that disabling it wouldn't lead to death (or infertility), or to serious debilitation (also likely to lead to death, in a non-benign environment).