This is all speculation on my part, but:
I actually think its tactical. Generally you have a duty to mitigate damages when a party breaches a contract with you. By saying Voom cannot survive without Dish and ending the service completely, AMC is for all intents saying its impossible to mitigate the damages we should get the full contract price. Starting new channels or incorporating the content into existing channels could be characterized as mitigation and would weaken their claim for full damages. I'm not sure if this lack of mitigation strategy will actually work, if I was Dish I would argue that there is still a duty mitigate. But I suspect that might be why we haven't seen anything done with the Voom content.
I think you're right. Dish losing their damages expert probably hurts their chances of making this case.
Voom will argue they have lost $3-$6 per month for every Dish HD subscriber over the life of the contract, and throw in some for lost potential revenue they would have had from other contracts since Dish's actions forced them to shut down. Subscriber projections will be based on current counts and trends.
The reality is they would not have had any other takers at the rates Dish was paying. If they did get other contracts, Dish's rate would likely have gone down due to MFN. If they didn't get other contracts, Dish's HD subs would be significantly reduced due to being over priced and the inability to carry other HD content.
Hindsight is 20/20, but you have to wonder what Dish was on when they signed the agreement.