I'm not sure how lock strength in folders has anything to do with fixed blades, considering that fixed blades aren't folders and don't have locks.
You are very much correct that an effective, comfortable sheath is an important thing to have, as a tool isn't much use if you won't carry it because it's uncomfortable to do so. But it's quite different from folders and their locks. Folders have locks in them, but fixed blades don't have sheaths in them (not any more than folders have pockets in them, assuming you carry folders in your pockets). A weak lock may be considered a weakness in a folder. A weak sheath has nothing to do with the strength or lack thereof of a fixed blade - the sheath isn't part of the blade, and it can be replaced or discarded at will without doing anything at all to the fixed blade itself. The sheath does not at all affect the performance of a fixed blade in hard use, while the lock mechanism does very much affect the performance of a folder in hard use.
So, a poor sheath is hardly a weakness for a fixed blade. The fixed blade itself is the issue - if it's good, then it's good no matter how the sheath is like. You can buy any kind of sheath you would like for a fixed blade from a sheath maker of your choice, and the problem is then solved. You can't buy a better lock construction to a folder, not without changing the folder itself. Some fixed blades don't even come with sheaths. The lack or poor quality of a sheath can be considered a weakness in the kind of a marketing "bundle" or "carry and use system" that contains the fixed blade and a sheath for it, but it's not a weakness inherent to the blade itself - exactly the opposite of locks in folders. What I'm saying is that if you like the blade, but dislike the sheath, then buy a better sheath, and your problem is solved.
Nitpicking? Yes. Important? Possibly. Offense intended? None whatsoever.