"proper" fillet knife is a really tough thing. Mostly because most fishermen who target specific species have very particular tastes, and those of us who just catch whatever is biting, well, tend to work with what we've got. For me, if I'm in a situation where I'm carrying all the gear and so a folder is a better option, then I'm less likely to fillet, since I'll probably be eating what I keep pretty shortly. Where as, if its a planned trip, I have no problem taking a fillet knife (I'm an odd duck in my locale with only one, most people have a cheap kit) and in that case, I actually care a bit more about the sheath and handle (rubber over-molding is fantastic) and so in all likelihood I'd go for a fillet knife that is more on the "commercial" end. I currently have a buck silver creek (thank you nameless salesman who sold it to my wife) and it does what I need it to do. Take that blade, get a pivot into it and make the handle as useful, and suddenly its a really expensive toy that I'm way more worried about, and honestly couldn't afford. I can justify a couple hundred bucks on my work EDC multitool and light, since it accounts for 50% of how skillful I look at my current job. Other than that, I'm in the one knife a year average group, so something that I'll use only once a year is a tough thing to justify. Where as, I loved the idea of the native salt, I'd had a native in the past and moved it on since I can't EDC a locker, but that moved right up into real fishing knife territory for me. Good cost, simple, does its job, and anytime I'm near the water, its one of the two knives I might take. I also have a sheepsfoot caribbean because of a work requirement needing a salt water knife with no pointy tip (hey, don't need a strong justification, any will do, work just happens to be a strong one for me)
I know its a long way of saying that things are complex, but I think that we as "knife-guys" really overestimate how many other people will like what we like. At the end of the day, targeting saturated markets is a harder gamble than finding new ones. Spyderco finds new ones, even if they don't always pan out, they do often enough that they do move more people into at least carrying something. If they could do it, I'd be keen, but again, out of what I want, what I can carry, and what I can afford, a folding fillet knife is very low on the list. That, and if knife guys are an odd bunch, you have to admit that fishermen are.... well, odder.
