And, other than true issues like floating versus sinking rates, shock absorption, etc., it hasn't been that long since most of us fished straight monofilament. Nylon lines were a huge step forward, held on and dominated for decades. Then, the "others" appeared. But, monofilament lines in "clear" spools were essentially invisible.
But, leaders have been around for a very long time using very tough materials, heck wire, for catching toothy critters in salt water, so it was inevitable that we'd see leader usage expand once multiple line materials appeared and became commonplace.
My guess is fluorocarbon lines would have disappeared from every shelf in America in, what, about 15 minutes after someone first tried to fish it with a whole spool of it on their reels. Especially the first fluorocarbon lines. Other "benefits" had to be sold and one of them is its refractive index being close to that of water, that it was even more invisible in water than clear monofilaments. It is, but to what end? And, is it sort of over-sold to us? I think so.
So, still to this day, a lot of us use fluorocarbon as short leaders on braid mainlines. For sure, some use it as mainlines. I use a braid to fluorocarbon leader on my spinning tackle for better performance (on my end) and a more natural looking presentation (on the fish's end). The pros will often comment, Aaron Martens comes to mind, that he used an 8 lbs. fluoro leader on day 1 of a tournament, that he dropped to 6 lbs. the next day because fishing was tough, the bite slow. He was admitting that he wanted the 8 for its strength, less chance of losing a fish, that he dropped to 6 not so much regarding line visibility, both 6 and 8 are, after all, invisible, but to further influence the natural action of his presentation. Here, it does make a difference. That little worm floating around on a hook looks more natural on really light lines.
Other than that, I'm like most here and I don't think most fish, certainly not bass, really much care about line visibility, it is never much for any line in typical water tints, and you could fish straight braid or mono or fluoro with similar results depending on the effects of each on presentations.
Brad