1. Honestly I have yet to see anyone discussing NV negatively tuning the older gen performance, only HKEPC members does this.
2. All cards decrease in value, you buy 1080 and it will drop in value in the minute you buy them. There's no right decision on this.
3. There is no direct evidence suggesting 1080p uses more than 4GB. There are reported cases does not mean this is a significant problem. My 980Ti pushes 3440*1440 60Hz without a problem, and you're afraid of it unable to push 1080p?
Buy a display card to play ur games, upgrade when you can't game.