I bought a couple of lightly used ukes of the exact same model; they had been recently restrung (before the sale) with specially selected Seguar fishing line. Although the ukes sounded okay with the line, there seemed to be something lacking. So I switched over to my stock fluorocarbons. On one of the ukes, I did the change all at once, and heard an immediate improvement, as if a new dimension and "sparkle" had been added to the tone. When I switched the other uke, I did it string by string, so I could more easily compare the new and old strings (especially the difference between the G and A strings, which would be the closest match in diameter and tone). Yep, the side-by-side difference was readily apparent, both between the two ukes and on the same uke. Of course, I also notice a difference between certain fluorocarbon brands—specificially, Worth browns and Fremont Black Lines have a different tonal cast (and feel), while Living Water seem a bit more balanced and "dialed back", and Savarez Alliance seem higher tension, etc. So the belief that all fluorocarbon strings are just fishing line and virtually the same is a provable myth, indicative that the person saying it has a tin ear, relative to the rest of us.
Which is not to say that, for what a person wants, they may not prefer fishing line, or nylon, or Sugars, Nylguts, Nyltechs, reds, SuperNylguts or what have you. It may be that they've just become accustomed to a certain string sound, and that's what they now consider the gold standard for uke sound. Or a person may hear so little difference, it just doesn't matter to them. That's fine, no reason they have to agree with other people's judgment (which may have become rather unduly set, even dogmatic—things do have a tendency to become accepted lore without sufficient basis or critical review).
You're right that the search for something "better" may have no end, but many of us have found that at least some searching has led to noticeable improvement and greater satisfaction. Changing strings is the simplest, most cost-effective means for improving the tone and sustain, and sometimes the volume, of a uke. Makers don't necessarily choose the "best" strings to match their ukes, they may simply use a supply of strings that they bought in bulk, or they may have worked out a sweet deal with the string manufacturer, or they may be pushing their own string lines (as Ko'olau does). A person may also need a different string type to better match how they most often play the uke. Too many folks believe that whatever came on their ukes is what they should continue using for the "best" sound, and more often than not, it just ain't so.