From what I've read, harvesting the biggest ones in a population whether it be deer, turkeys, or fishes, makes more room for a greater number of medium sized animals to move up into the largest range. Think of it as a pyramid, with the biggest animals being on the top. The alpha fish consume a greater percentage of forage relative to the number of their group. By harvesting more of the largest specimens, it makes more room for a greater number of medium fish to get bigger and move up to the top ranks. Over time, there will be a greater number of bigger fishes in the body of water- assuming you are dealing with a good sized body of water.
Most biologists agree that with crappie especially, they can breed in such numbers that it is almost impossible to fish out a larger body of water.
I also think that the decrease in populations is more a factor of more people fishing and not better technology exclusively. 5 years ago, I could go to the smaller lakes in my area, and see maybe 1 boat in a particular cove. Now, there are 2-3 in said cove. All us boomers are retiring and have both the time and income to purchase expensive new tech. Before LS or even DI and SI, I saw populations of fish and quality of them decline slowly.
LS just makes locating fish more efficient, and it does not make them bite. We are becoming more educated as to crappie habits with LS. As to people keeping more limits, over the 60 years I've been fishing, I have seen that the poorest folks usually keep the most fish. That's OK. Fishing to eat was the thing you had to do in the old times. Poor folks cannot afford LS or other modern electronics to bring home the catch. Fish your own way of fishing. As long as it's legal, I'm OK with it. Now someone else can use the soapbox.