I agree with most of what you wrote in your post above about the problems Republicans have with their current situation and the country's demographics. They would have trouble winning any national election at this point and especially in the future.
It's not that I am really concerned about Hillary losing - though that is a possibility. As I pointed out she has a lot of weapons at her disposal she can use to reverse the current poll trends or at least mitigate them. However, it is distressing to to watch her lead in key states whittle away, if only because the wider her coat tails the more likely it is that we will take control of the Senate and at least lessen the Republican's control of the House.
Rather than being complacent, all of us on DU should be doing everything we can to GOTV, if only for the down ballot races.