Well, first, I don't consider Smallville an example of an adaptation that bears little resemblance to the source material. (Which is a subjective call, but I've never claimed otherwise. It's art. It's all subjective.)
That's because you're looking at it from the tail end.
Smallville started at a time before superhero movies ruled the box office, when comic-book-based projects were still considered something of a disreputable niche. So the original intention of the series was to reinvent the Clark Kent story without any comic-booky elements at all -- "no flights, no tights" -- and present it in a form palatable to mainstream audiences, something more in the vein of
Dawson's Creek or
Roswell. The same principle as the same network's short-lived
Tarzan adaptation that was reinvented as a New York City detective drama -- take the characters and premise, but strip away all the genre elements in favor of something more conventional and crowd-pleasing. The express purpose was to transform the concept into something palatable to audiences who had no interest whatsoever in superhero comics, who didn't even know the show was inspired by Superman (and there were indeed fans of the show who did
not know that).
But as
Smallville went on, it changed massively in its approach. It ran so long that it drained its original premise dry, so it had to draw more ideas from the comics in order to sustain itself. And by that point, superhero movies had become more popular and respectable anyway, so the show didn't have to try as hard to conceal its comic-book roots. So in retrospect, people look at it as a heavily comics-influenced show. But I'm talking about the original intention of the show in its early seasons, when it aggressively avoided anything that even hinted at superhero comics.
And that very transformation within a single series highlights what I'm saying. Art is a continuum. You can't define impassable walls between different kinds of storytelling.
Secondly, again, I don't think anybody's saying adaptations that bear little resemblance can't be successful or good or high quality. Whether they're good is a separate matter from whether they'd be better off as original works instead of latching onto their vestigial IP connection to the source.
The problem there is making it a blanket generalization -- whether "they" would be better off, as if every one would have exactly the same result. That is utter nonsense. As I said already, quality is not determined by category. If it were, then everything of a given type would be equally good or bad, and that is obviously not true. Look at all the cases where one movie or show is a huge hit, and a dozen attempts to copy it are flops. It's not
what you do that matters, only how well you do it. The ones that are "better off" are the ones that are
done well, and whether they use existing character names or change them is a trivial consideration by comparison. That's the surface, not the substance.