I have been using Sapphire HD 4870 for almost 10 years now. It was a really good GPU at the time. I'm thinking of upgrading to a newer GFX.
I like to make electronics purchases that last for a long time, and are rather slightly overtuned for my needs. I hate buying electronics, and then having to replace them in <5 years.
I don't play computer games much anymore. But I have a 2560x1440 screen that requires a good GPU.
Games that I play would be: diablo 3, wow, lol, civ 5 and HoTS.
I know that they don't require very effective GPU. I don't play any newest games with big requirements. I still like the idea that my GFX would last a very long time without needing to upgrade. So if there was a Diablo 4 coming (in 5 years), I wouldn't have to buy a new GFX for that.

RX 580 seems to have more delivery issues atm than 1070. I could buy 1070 right now, but can't say the same for 580.
1070 is >400€, 580 is 300-350€.
Which of these GFX have a better price/value ratio?
Would 1070 be too much for my needs?
I'm really looking forward to the long-term value.

Any input appreciated!

You want to buy an expensive GPU today, that'll be obsolete and really cheap, in a few years, to save you from having to buy it when it's cheaper --- because that's the way you are.

Ten years? Surely you'll upgrade the machine in that time, so this is an odd question. You'd get far better value buying something more suited to your needs now (a previous-generation nVidia card would still be way more than you need, surely, and last for several years, at half the cost of a 1070).

On top of that 1070s etc have vastly inflated prices at the moment because of people mining Ethereum and other cryptocurrencies. It's a terrible time to buy a high-end GPU.

RX580 is better value but may be too weak for 1440p gaming unless you have low demands on FPS/video quality.

1080 (maybe even ti) weirdly enough may be the best value out there now because miners don't want them because they aren't as cost (including power) efficient. However, 1080(ti) can still pay for itself in about a year (I am factoring some inevitable drops in mining efficiency).

At least that's what I told myself when I got my 1080ti.

Get a ti and be happy imo.

Getting a ti for 1440p gaming soonish fwiw.

for 1440p get the 1070