Did people really think your Draenor garrison would carry over? It makes sense that anything you do in your garrison is likely to be obsolete next expansion. It would make no sense for your main hub to be in Draenor anyway once we've moved to a different continent. They didn't say the entire concept of garrisons would be scrapped, they said that they would use the garrison technology to continue, just like farms -> garrisons -> something more advanced.
If your existing garrison progress carried over, then anyone starting a new character would be a full year behind catching up with their garrison and that is just not good at all.