First off my comment was slightly tong in cheek and secondly at no point have i said 'Most' people have upgraded, questioning how much market is available to a product which is late to release and may only offer the performance of a product which has been available for some time is quite valid, a finite number of people are available to market a product of such price to, limit this pool further and you limit your potential for sales.
Clearly sales of video cards do continue through its life-cycle..... However i imagine a reasonable proportion will be purchased within the first few months, a steady turnover of product will then continue though its remaining life-cycle, this is fuelled by sales initiative, such as price cuts and promotions designed to tempt people who otherwise would have not considered such cards.
The thing is not everyone is following graphics card releases,to even know if Nvidia or AMD have something new out.
In most cases,people upgrade for the following reasons:
1.)They bought a new game which needs a better card
2.)They upgraded to a new monitor
3.)They finally saved up to get an upgrade they need
4.)They finally saved up to get a new system
5.)Their old card has gone kaput
6.)They need new software features not supported by the older card(say an CUDA or OpenCL update which does not run on an older card)
7.)They have the funds to buy new shiny
The whole point is,look on Steam - most people are not running GTX980TI/Fury X/GTX1070/GTX1080 level cards.
AMD has no doubt lost potential sales from not having Vega out last year,but in the scheme of things even GTX980TI/Fury X level performance is more than what most people have and there are still plenty of new games being released which might push people to upgrade.
It also applies to the RX480 and GTX1060 too,and this is despite the GTX970 and R9 390 being out for yonks and not being much slower in a number of games.