The current EOL set market is dealing with being a victim of its own success. For any old timers or middle-aged timers even, the exact reason you (we) enjoyed the profits we did is why a lot more people are in the game now. Without those profits, we wouldn't be in this situation. How many people checked out eBay and saw a retired set's selling price and thought "Wow, I have to do more research into this....had no idea LEGO sets could be this valuable" That's exactly how I started, just so happened to see what a Cafe Corner was going for. That was before the days of BP too which has been a primer for any new investors to help avoid a lot of those early growing pains and has helped accelerate the field of competition. Not complaining about it though, I enjoy being part of this site.
Historically, any industry that has a low barrier for entry is going to have to face heavy competition the older that industry gets. Let's face it, it doesn't take a scientist to buy and sell LEGO, just money, space and a decent work ethic if you want to have some success...we didn't have to go to school for 10 years to try and cure cancer, we're just reselling plastic.The barrier for this industry is fairly low.
Now, that's not the end of the world, just a shifting of expectations. It will reach a point where the curve tops out and you won't have all these new sellers coming in. Unfortunately, this is also tied to profits, but this time it's because the easy money is gone and its not attractive to people outside looking in. You also will have some people leave for greener (true or perceived) pastures.
I guess the only question is if we are near that point of "topping out". I would say with all these really nice exclusives just retiring, that this business will still be getting a lot of curious new eyes looking into it in the next few years, so there is going to be a good amount of competition still coming in keeping the Plastic Rush alive.
Enjoy the profits that are still there in these middle years of growth, just keep expectations in check.