its cheaper to buy one card every release and sell the old one than it is to invest in a second card for an sli setup that will eventually perform worse than a single card, as he said the 780ti hardly keeps up with a 1060 today, imagine spending $1200+ back then for sli 780tis that now perform worse than a 1080, and are massively overwhelmed by the 1080ti, the same will be true for the 1080ti in the future
if you must build a pc now go for the 1080ti, when its replacement is released sell it off and buy the new one, and so on etc etc