So a couple of points I'd like to raise..
1. Will this affect previous generation cards through drivers? For example, say I go tomorrow and pick up 4x Titan X's, (which AFAIK have always been able to do 4-Way SLI for games without issues other than poor scaling), will this come along as a driver update and limit that setup to only 2 cards working for games? Or, say I get 2x GTX 690s, what will happen to them?
2. Multiple card SLI would be a lot more popular in general if it weren't so expensive(my $.02). Most of the general population of PC gamers can't even afford 1 GTX 1080, let alone multiple. Now that 4K is a lot more mainstream than in the past few years, more and more computing horsepower is needed, the logical thing would be to continue support for 3/4 cards and focus on getting flagship cards to lower price points(edit: where they could sell more volume). I think it makes sense especially considering higher resolution and refresh rate screens are hitting the market that are able to max out even 2 cards. (IE, 5K, 144hz UltraWides, 8K soon, etc.)
3. With this all in mind, I wonder how big AMD cards will go later in the year, especially if they support 3/4 Crossfire. I could see the big money gamers and enthusiasts getting 4 flagship AMD cards to drive high resolution/refresh rate/benchmarking setups, assuming that they will support 3/4 card Crossfire.
And finally, I do wonder if this means the end of dual GPU Nvidia cards, even if they were to release a GTX 1090 (dual 1080 GPUs), it would pose zero advantage over the 1080s, since you couldn't add a second to it.