Considering what they have planned and all of the variables that, as of yet have not come into play, a broad release window such as that is very much justified IMO. At least it gives us some idea of what to expect.
Being announced in 2012 and having the possibility of releasing in 2021 however? It is mildly irking to consider that we are getting more and more instances of huge development cycles and unexpected delays. It speaks to the increasing technical aspects of games as well as the magnitude of scope rising. In this instance it also speaks to the unexpected success of The Witcher 3 (and thus the unexpected amount of resources dedicated towards providing continued support for the game, as well as the increased want to one-up said game).
However they seem very gun-ho, and I doubt that it will release after 2019 tbh. Especially with the possibility of 800 staff.