We can't assume any of these sets are great sellers, true, but your point of LEGO extending set lives to 6 years was based off of one set, the 10188. There have been over 10,000 LEGO sets produced, and even if 10 or 20 had a 6 year shelf life, that is an outlier. The 10188 is an exception...for now. In no way am I knocking the 10188. I have pointed out many times that bang for your buck, it is one of the best sets LEGO ever developed and sold. Maybe LEGO is changing their philosophy on retiring sets. Time will tell.
My original point was that by retiring LEGO sets, it makes them more valuable than if they were never or rarely retired. Sure, it helps the investors of the world as well, but most of all, this value after EOL helps LEGO itself. If an item is perceived as valuable after it is retired, more often than not, the same would be true when the item is being sold by primary retailers. It is much easier to sell a $400 LEGO set to an AFOL if they believe that the LEGO set will be worth more money one day than they paid for it. How many $400 boxes of plastic would LEGO sell if the pieces became almost worthless after opening the box like many toys? Not many. The primary and secondary markets compliment one another and the retiring of sets is a key component to it all.