It would not be cost effective to come out with an actual new vehicle that often. Being as though they have to design, produce, revamp assembly lines, and inventory parts, warranty them for long periods of time. That is part of what GM's problem was in the past, making to many of the same vehicles, and spreading it to different brands.
The purpose of Special Editions is not necessary to gain sales, but to continue selling owners. If I own one for 3 years and nothing has changed why would I buy a new one? I managed a HUMMER dealer for years. HUMMER started the twice a yr Special Editions. The people that would buy your SE were current owners that happened to be in for service or whatever, and saw it sitting on the floor. A good sales person, and a good trade value, and its a done deal. It takes GM 2 yrs to put a new paint into production. Special Editions are usually sent out to an upfitter for the different features added. Believe me Wheels and paint sell vehicles, it seperates you from every other one the same on the road. Those type cars are bought on impulse very frequently. GM knows it has to keep its current owners in its cars down the road. That is why you don't get all the features in the first couple years. You have to have a reason to trade it in a few years later.