It is simple economics that if you can produce a product that sells 100% of it's production numbers and has a reasonable mark-up included in it's cost that is enough to cover the R&D necessary to produce it then there is no reason not to make said production model.
So in essence, if GM knows they can build a Z/28 that will allow them to make a profit not just break even then they will. Seriously if you are so bent out of shape about if they are or aren't going to build a Z/28 then you aren't looking at this objectively.
What I mean by this is that if you really want a Z build one don't wait to see if they are going to build one. Buy a base model and put an LSA in it...if it were me to do, I would buy an SS and put a 427 in it and call it a day.
GM, is trimming fat right now, what I see them doing is waiting to see how the Camaro is going to sell on a small basis. Once they have made some money back on the car they can think about variations, and I have faith that we will see some cool options in the coming future. Just hold tight, 5 years ago no one thought we would even be seeing a new Camaro.