Pretty sure it's been said many times in the past that they have suggested the 'possibility' of it being added to those cards in the future but I wouldn't hold my breath.
Also if you think about it, them pushing something like this out to older cards asap could effect sales of newer cards and look.. at the end of the day any company wants people to buy the newest release (when they appear).
Also if you have a 5000 series card do you actually feel like you are being 'forced' to buy a 6000 series because of the resize bar???!! Because tbh it's not that huge a boost for most games. You can check online and there have been many tests by the likes of gamer nexus etc on it and the the increase is small enough and in some cases it's actually gives a slightly worse performance.
It'd be a nice feature/option to have for the 5000 series cards but it's not a make or break feature
