Some decisions taken when designing the X might have been a mistake. But it would be too simple to blame all onto them. While initially, the X was planned as a minor bodywork modification of the Model S, the crazy success of the S gave Tesla the money and the time to make it a larger refinement of the S platform. In the meantime, the S also kept being updated.
So the X is rather still part of the S platform and the real cheaper car will still be the Model 3. In that sense the Tesla plans did not change much. While not many X were delivered in 2015, they seemed to have started mass production literally in the last weeks of the year. They should be building several hundred of them per week now.
That the X is even more expensive than the S is not a bad thing, considering that they have about 20k orders backlog. As long as they can sell any S or X they make, there is no hurry for the "cheap" model. Revenue per car is more important that total car number sold.
So the X is rather still part of the S platform and the real cheaper car will still be the Model 3. In that sense the Tesla plans did not change much. While not many X were delivered in 2015, they seemed to have started mass production literally in the last weeks of the year. They should be building several hundred of them per week now.
That the X is even more expensive than the S is not a bad thing, considering that they have about 20k orders backlog. As long as they can sell any S or X they make, there is no hurry for the "cheap" model. Revenue per car is more important that total car number sold.