Since they come up with 1 exclusive each month on average they probably need to retire some stuff to achieve a sustainable steady state in production and marketing. After all not many exclusives retired this year, I remember only FB, WV Cottage and SSD, the latter still available in Europe. The others are unconfirmed.
Meanwhile we had the 3xE, PR, Sandcrawler, Mixer, Sea Cow, Tumbler, Mini, Slave-I, Simpsons, WV workshop come out this year. 10 sets. LEGO has to maintain balance somehow, the number of available exclusives shouldn't go to infinity.