I gave up on FAA awhile ago, at least to be anything more than a pretty good player with maybe a hot run at some point (e.g. Dimitrov in 2017). Not sure what you mean about "clinging nails."
And again, consider the difference between "floor" and "ceiling" as a span of likely outcomes. I think Fils' floor is higher than Rune's at this point. Fils, at worst (barring injury) is going to be a back-end top 10 player - a guy who wins a bunch of 250s and 500s, and maybe several Masters. Rune, at worst, is going to continue flailing and be a cautionary tale.
But yes, Rune has a super-high ceiling. If he puts it together and gets his head on right, he's going to be the third of a new Big Three. I don't think Fils is capable of that.