This isn't "my point," this is how the field of evolutionary biologists look at traits. If what I said was false it would mean there's no such thing as natural selection. We may not understand how a trait is beneficial, and we may be incorrect about why a trait is beneficial, but if it has a cost and lasts through evolutionary time then by definition it has a benefit.

>it could be explained as the incomplete or nonexistent development of something that is complex that is beneficial.

If it has a cost but doesn't have a benefit right now, it's going to get weeded out via natural selection in favor of variants of the organism that are not wasting the time and resources on that trait.