Even disregarding anything but mental and physical health, the consequences are significant and quite real.
No, they don't need to become the gatekeeper of all "bad things"(tm) the same way they protect us from accidentally gazing at a terrifying nipple, that would be preposterous, but they could probably try a little harder to not act completely opposite to their users best interest as often as they do.
Especially when that happens to be a significant fraction of all the people on earth, that's probably not too big of an ask?
What is this alternative you speak of?
Optimizing for engagement tends to favour extreme, simplistic, and highly emotional viewpoints. In other words, it caters to human nature. This tendency is harmful to rational discourse, regardless of whether or not you happen to agree with any given viewpoint.