For those concerned about the single player games. I don't see how this will affect them.
It has to do. Geography of the province matters so much in a TES game it determines the narrative flow.
Even when limiting to 3d times, the technological advancement of graphics might justify by itself a complete overhaul of Morrowind/Cyrodiil provinces. Because visual ideas could not be properly implemented in those days.
Skyrim on the other hand is another story. Sure, graphics will improve but already, certain elements are secured and Skyrim geography can not be redone as much as the others.
As I understood it, the online project will encompass the whole continent and will be exclusive to PC, meaning that they are done with console limitations.
The expected result is that they are going to give a face or visuals to every province geography, and visuals that will last considering the graphics technological stage.
The end result is that you will already know the visual feel of any province they are going to use for the next SP TES game. They will probably play with the time shift to show the evolution of the continent but many geography landmarks have to be preserved to maintain consistency.
This might be checked when compared Skyrim and Skyrim online when the game is out.
Obviously, there will be a split between players with players who will prefer to discover the geography of a province originally through a TES SP game while others will rejoice about finding out the multiple differences showing the evolution of the continent.
But it remains that this online endeavour will influence deeply the TES SP series as geography matters so much in this series.
Another consequence is that they might also speed up the developpment of TES SP as the bone structures of a province are already developped.