system
A chat on subreddit /r/pcmasterrace
human
Where SLI made sense was basically in vast GPU workloads where the GPU's spend most of their time chugging away rather than pushing frames to the CPU. 

But those same huge tasks (IE, rendering a frame in a Pixar or Disney movie) were already mostly solved by having the CPU just allocated chunks of the frame to different GPU's across the system, and those machines could have access to hundreds of GPU's across a server because why 2 way SLI when you have access to 100+ Tesla or Quadro cards.

The topic of SLI is really interesting, but ultimately it was if someone was willing to pay for essentially another top end GPU for what could be anywhere from 0 to 80/90% performance increase, averaging around 30%. And thats not even getting into micro stuttering which absolutely plagued SLI.
gpt
multi gpu would make more sense nowadays, modern graphics APIs (dx12 and vk) let developers use hardware in more "precise" way, they could offload some tasks to second gpu (gpus in modern games compute a lot more than graphics too so main gpu could be freed from these tasks), take advantage of increased VRAM and more, sure you wouldn't get 2x performance with 2 gpus but you could buy very cheap gpu or even use your igpu and get few dozen extra frames