The logic here is flawed.
Spider-man, Daredevil, Punisher, and Fantastic Four all have similar origin stories, handwaved origins that make no plausible sense, and exist in a medium which is, by default, fantastic.
Punisher is almost always grounded in most of his stories. They barely fit in when you realize in the arc where he deals with a mob boss the city of New York was being flooded by Namor using bullshit powers and a kraken.
Daredevil’s most popular runs, as well as most of his history, is very grounded. The fantastic is largely only in origins, not in his world nor how everyone behaves in it.
Fantastic Four have met the omnipotent God twice. Its not even really all that worth mentioning given the amount of bullshit they encounter and go through daily.
Spider-man straddles both, mixing grounded life with super science and magic before looping around to show folks as mundane despite all the extras.
Grounding Fluffies, indeed trying to ground any fiction or find a grounded place within it, is not anathema.
As I said, Fluffies as an idea are not impossible. As presented? Just pseudo-science bullshit handwaving.
But that doesn’t require you to go for broke in eliminating the fantastic, nor in going as far as the dial goes on it.
After all, by definition the reference to grounding fiction is trying to create realism in the unfathomable. Otherwise it would just be modern.
Adding in a plausible business model, legal ramifications, cultural evolution, scientifically sound ecological impact, and consistent anatomical features doesn’t mean I’m missing the point, in that there isn’t really a singular point. Saying that its a pointless and simply wrong endeavor to work on basically zero-sums my intended work, leaving me only with exploring how utterly worthless all forms of life is and the question of how Fluffies would affect the supernatural existing within folklore.