As to their effectiveness, unless everything has trigger warnings it’s not going to do much difference, people will still be exposed to the things that traumatise them. I’m not sure that’s a bad thing, you need to get used to living in the world sooner or later.
In short, your ideas here appear to be shaped by your politics, and they simply doesn't match up to mental health best practices for effective treatment.
Also, it's gone beyond it's originally intended purposed and begun to be used with any sort of "extreme" content, e.g., horror/gore/jump scare videos etc, where it serves as an inducement to watching, not a warning against it.
But aside from these, in the original intended context, I don't really see the behavior you mention. Maybe it's the case that a small minority are the caricatured smug virtue signaler you portray, but if so you are painting their entire use with the brush of an extreme minority of an example.
Visit a depression support group forum. You'll see just about any post with suicide content with a trigger warning. Why? Because while it's a common issue among the depressed, suicide is also contagious in a way. People already at some risk are much more likely to attempt or succeed when exposed to content of that nature. It's not uncommon for there to be a slight uptick in suicides after a public figure commits suicide. The second of two suicides within a week from survivors of the Parkland shooting may very well have been influenced in this way, as might the third suicide of parent of a Sandy Hook victim that committed suicide a few days after that.