And the rest of the stuff? Memory, motherboard if they change the architecture like you said? Not saying it's alot pricier but having build serveral (and uppgraded) I have never found this to be true. And not once have a GPU allowed me to play the latest games for 6 years unless you buy the top of the line stuff.
You don't replace your motherboard or memory until you replace your cpu
People who bought an i5 2500k (190 euros) and p67 motherboard (100 euros) and 8GB of ddr3 (60 euros) back in 2011 are still using them and if they want to they'll keep using them for another 3-4 years as the cpu is still several times more powerful than the ps4 cpu; and on the pc cpus haven't gotten much better since then (might change with amd's new zen cpus at the end of the year).
If they're like me they'll probably still be using the same case (mine's from 2009 and I'll keep using it for another 10 years if I can, an ATX case is an ATX case it doesn't age, it has no moving parts to break) and power supply.
Tbh I find it sad that cpu performance progress has stalled almost completely , but the advantage is that you pretty much never have to buy a new cpu once you bought a decent one.
I'd personally like it much better if I still had to buy a new cpu every 3 years
as that would mean games would be getting bigger and more advanced every 3 years thanks to large cpu performance improvements.
Graphics will continue to be better, yes. Even when they are cutting edge innovations like VR or higher res screens are going to call for better graphics and leave room for improvements.
My point on diminishing returns is more focused on game design. Power is not enabling different types of games anymore, making it easier to scale games down/up across different hardware specs.
That's what I mentioned rainbow 6 siege : the destruction of all the walls, not scripted but physics based, allowing you to create your own lines of sight (of dynamic sizes, be it bullet hole sized or blowing half a wall away) + the realtime dynamic GI needed to make the visuals work. That definitely wasn't possible on last gen
Crackdown 3 : Being able to dynamically demolish entire cities (and have the debries and building parts still be physical objects you can further interact with , walk on and use as cover) certainly wasn't possible last gen (and I seriously doubt it even possible this gen the way MS demod it)
Battlefield 3-4s destruction is child's play in comparison
Star citizen: the multi crew system from Star wars galaxies expanded into being able to EVA (leave the ships into zero gravity) and have proper ship physics and destructibility etc all at once in multiplayer has never been done and wasn't possible before
Those are exciting new gameplay applications.
Sure they don't happen as often (mostly because the transitioning to 3d in the late 90s opened to many new possibilities at once) as they used to but I'm sure that developers still constantly have to can ideas and concepts because they're still not feasible on modern hardware