Nvidia doesn't future proof their cards lately. They are designed to run for 1,5-2 years and then you are left with no driver performance improvements, to little VRAM, and not able to run newest Gameworks features like VXAO in Kepler, or e.g. Hairworks in the first weeks of Witcher 3 release until the petition from users worked and playable framerates were activated. Pathetic. And after 2 years you have to buy another GPU therefore increase the company stock price.
You can also buy Radeon, just stand beside, get the same performance now, and watch it overtake price point competitor one by one over time. It's sometimes a bumpy road, and you get worse frimetime consistency, but it always gets better via driver updates eventually.
There is poop on both sides of the fence..
I don't think that they made the decision for the purpose of intentionally crippling the card. Also, even though I was pretty surprised at the announcement, there has not been really huge performance issues from it. I personally would not buy one because we don't know how much drivers are keeping the performance of the card in line, but my guess is the decision was made with regards to cost/profit on the card itself.
I am not sure that it having 512mb more of full speed RAM would make any real difference in longevity. And I would posit that the reason we see gaps between older and newer products in Nvidia is partially due to architectural changes. Where as GCN has just been evolved on from the AMD camp. So currently we are seeing a benefit of this in that drivers for the 390x also help the 7970.
However as anyone will tell you, AMD is not without their driver flaws. Only recently have they fixed the issue that Karlitos and other 3 monitor users had for over 6 months.
On topic:
I don't think the card benefits from 8 GB of RAM. What game does it have the horsepower but not the VRAM for? If you are going to fork out money for more GB, why not grab a 980ti or 390x. However, even those cards run out of steam at 4k where 8GB is a concern.
I don't think we have a card out with the horsepower to warrant 8GB unless you include SLI and Crossfire setups. Even then it isn't great.
290x 8gb vs 4gb
http://www.tweaktown.com/tweakipedia/68/amd-radeon-r9-290x-4gb-vs-8gb-4k-maxed-settings/index.html
Would you pay for more VRAM for that? Personally I would pocket that money and wait until a card comes out that can handle it.