When the old walleye start dying off you'll see new ones move in or another species take their place. If there's an open niche, it will be filled. The original walleyes used for stocking looked like hairs when they were released, they grew up. More recent seining shows successful recruitment. Remember, seining isn't meant to be used quantitatively, it's to demonstrate spawning success in this case. It's a big lake, walleyes fry will be found in many many places not sampled or even suitable for seining.