True they have done it before. When the iPad 4th gen came out they scrapped the 3rd gen and kept the 2nd gen, but the iPad 2nd Gen was only about 1.5 years old, and the 3rd Gen was kind of a misfit.
The iPad Air 2 will be 2.5 years old if we are talking about a spring release. Then to keep selling that for another year or more, I don't think Apple will do that.
The Air 2 is still a remarkably good tablet though. In fact, all things considered, it's arguably the best value in the iPad lineup right now. it may make sense to keep it as the entry level 9.7" model.