I don't think people were actually banging on the 290X 8GB being "future proof", but more like at £299.99 the card is ridiculous good for value, and would provide a "ease of mind" without having to worry about games being poorly optimised/ported and using ridiculous amount of vram, or may it be due to extreme high-res texture or whatever.
If the 8GB 290x was at £350-£400, nobody would really recommend it for 1920 res.
"We have potentially a truly future proof 1440P & 2160P card when your running them in at least crossfire, 3-way recommended for 2160P. If your gaming at 1080P then a single card should be more than future proof for years to come even on the poorest console ports which could be using around 6GB of memory"
Not the best example i know, but just read through the epic 970 thread, about 2 thirds in the 290x 8gb becomes the defacto Gpu of choice, i could spend all day quoting people and threads with the words 'future proof and 290x 8gb in them'