We'll still need at least 2 GPU's in SLI / crossfire to get anywhere near 120 Hz refresh at 4K, and that need will maintain itself for quite a while if for no other reason that games get more demanding over time, so even if we have GPU's twice as fast as anything out now, the latest games out by then will be a lot more demanding too...….
Never mind 5K ( 5120*2880 ), or 8K (7680*4320), which will destroy any hardware for the next several years, especially 8k being that it's 4x more demanding than 4k so here we are all happy to see 60 Fps happen at 4K. when in both fill rate and memory bandwidth terms, 8K is 4x more demanding so a given game that does 60 Fps at 4K, could drop as low as 15 Fps at 8K.....It's unplayable.
The only option is to use the same solution AMD does for their CPU's and put multiple GPU dies in the same package and each has a connection to the HBM memory chips and between themselves ( infinity fabric )….We've only got the 7nm fab process in terms of GPU makers actually showing working silicon using that process and stated to be released next year, and below that is nothing but hot air and will take years to see anything.....3 to 4 years if not more.