You really think this card is gonna last forever? It's not, when the 14/16 cards nm comes out the Titan will be a waste of money. Get a 980 ti and overclock it instead, it overclocks like a god and will beat the Titan easily when it's overclocked. Just look at graphics cards from 5 years ago, they're horrible now. This world moves fast and no card is totally future proof.
HeadFoot is right too, about the reference design. It's shit. Trust me, i have a Nvidia card with the reference design. 86C and it sounds like a vacuum cleaner. Don't get it.
FOREVAR CARD isn't going to happen. Moore's Law is just like that.
Buy a good card now, and upgrade in a couple of years. Don't blow your money on 12GB of GDDR5 for "future proofing", when next year's cards will have 8-16GB of HBM2 with 14nm shaders and make it look like an antique.
This could work, but only if you run old software. Never buy another new game, just run old games, forever.
But if you ever want to run a new game in the future, you risk seeing your hardware become obsolete.
all you said it s true for a custom build pc, where u can change MB/CPU/... but i have to make some clarifications:
1) i have another Custom build pc watercoled and that is gonna be always upgraded. This discussion is not about it !
2)
This gaming setup is to consider as a console: not upgradable, because it is a pre-made pc and i can only change the
gpu, i took it for design and i m not going to sell it never. When too old it s gonna be used for retrogaming (as you said!)
The other parts of the pc cant be changed and are probably going to limit a too powerfull gpu ,so i have to chose 1 gpu to remain forever in this pc without be limited by the rest of the config 
--------------------------------------------------------------------------
̶C̶u̶r̶r̶e̶n̶t̶ ̶C̶o̶n̶f̶i̶g̶ Forever Config
Screen : cant be changed [Alienware OptX aw2310] (
1080p 120hz)
Keyboard : cant be changed [Alienware TactX Keyboard]
Mouse : cant be changed [Alienware TactX Mouse]
MB : cant be changed [Alienware MB - msi rebrand and mod]
CPU : cant be changed [3970x]
Ram : cant be changed [32gb]
SSD : cant be changed [1TB]
HD : cant be changed [4TB]
GPU :
can be upgraded [2x7970ghz]
is pascal gonna be limited by the other parts ? cpu can handle 5ghz