I ran SLI'd 8800 GTX cards for years. It was only when I bought a game that needed DX10 or 11 that I upgraded to a GTX660... Then last year upgraded again to a 1060, because I wanted to play on high and ultra settings of some newer games.
Basically what I'm saying is, it depends on what becomes your limitation. Is it an API limitation, or a settings limitation? Most people can use the same card for many years and not be bothered, others need to be able to play the latest triple A games on super ultra max high settings with a 1000FPS...