I believe it was Isaac Newton who stated something to the effect that if he saw further than others it was because he stood on the shoulders of giants. Up until the mid-19th Century most great painters and sculptors and academies placed little value or virtue in original subject matter, but rather the ability to reinterpret or, "reimagine," subjects from classical mythology or the Bible.
Of course it is far more complicated than that and you won't have to look hard for exceptions, but broadly speaking, in times of high illiteracy, a painter, sculptor or stained glass window maker wanted their audience to immediately recognize their subject matter, highly disincentivizing clever originality. Again, generally speaking, it is a hallmark of modern, abstract, non-representational art that wishes to leave the viewer confused, disoriented and their art subject to individual interpretation.
There is nothing inherently wrong with the broad concept of reinterpreting or expanding upon old stories and old characters - it is how our culture is transmitted. Within that broad concept however, some reinterpretations may be very well done, while others suck. Take for example, from modern pop culture, the whole Batman mythos. Both the Christian Bale and George Clooney versions were reinterpretation and reimagining of a long-existing character. One (Bale's) version was extremely well done and will long be recognized as great cinema, while Clooney's interpretation will soon be forgotten by those who haven't done so already.
I agree with much of what you're saying... we stand on the shoulders of giants... in all fields.
That said there's a difference between writing something old in a new and better way and the whole cloth theft of a fictional character created by another writer . It crosses some kind of line with me... But yeah - in many cases when it's done it's done better. Often worse...
Years ago a friend who was teaching writing told me there are only 7 themes for novels ... should have asked her what she thought they were...