While not a factor in the case, today many judges would consider the effect of their decision on other cases. Here, awarding the cost of performance might make coal mining companies more likely to actually do restoration work. We will see that there are exceptions, but the main purpose of contract law is compensation, not punishment of those who fail to honour their promises. Arguably Sally Wertheim is consistent with both positions. As we shall see, the principle of Sally Wertheim is useful and does help decide many issues, but not this one.
Tenant fails to do as promised.
BUT - Absolutely clear windfall gain to Owner. Work cannot be done. No court is likely to apply compensation principle in such a way as to award cost of performance in this situation. Defendant failed to build wall. BUT 1. may have fulfilled aesthetic purpose for plaintiff 2.