So, the G2 series was a very good power supply, made for EVGA by Super Flower and based on the Leadex platform. You'd be hard pressed to find anybody who is even semi knowledgeable about PSU technology that would indicate otherwise, however, having been purchased in 2015 that unit is nearly ten years old. That unit originally had a 7 year warranty that could be extended to 10 years if you registered it with EVGA within 30 days of purchase. I'd likely trust the unit with any hardware that was of a similar age that had been put into service at the same time as that PSU was but if you are going to spend money on a new graphics card it is an extremely poor idea to pair it with a nearly ten year old power supply no matter what model or how good that PSU is. I would recommend against it even if it was a high end Corsair HX or AX model or a Seasonic Prime Titanium, at ten years old.
And beyond that, at the age of that PSU, it would be an investment in the rest of your system's continued health in any case since as you have said it still does what you need it to and you'd like to see it last a while longer. Even the best of PSUs become a liability at, or especially beyond, the 7 to 10 year mark. And for most models, which aren't as good as that G2, I'd change that to the 5-7 year mark.
As for the graphics card and resolution, if you are not gaming there is literally no benefit to you that could be discernible running 4k/144hz other than the pixel density and additional real estate of a high resolution, larger physical size panel. The 144hz refresh rate gives you nothing tangible if you aren't gaming. For productivity, general use and even watching movies or video, it would be of far more value to place your emphasis on resolution and color accuracy.
Regardless of all that, the fact that you want a graphics card with more modern capabilities is understandable to some degree, although, the RX 580 you have now can easily do 4k on both HDMI and DP, it just won't do it at 144hz because it does not have modern enough standards, but what you haven't offered is whether you have any preference as to whether you would prefer the card to be AMD or Nvidia based, and specifically what type of output/connection (HDMI, Displayport, etc.) you plan to use so that we can be sure the card chosen supports the standard that will give you the resolution you desire AT the refresh rate you desire. There are some fairly modern cards that don't use the latest standards, using older HDMI and DP standards instead, so knowing these specifics might be of importance.
Another consideration is your case. We have no idea what size card you can go with because we have no idea what case you have. We don't know if you have a SFF RX 580 or any specifics in that regard and that might also be important. Granted, most lower end or entry to mid level cards are usually of reasonable length, but reasonable is rather subjective especially if you're using an older case with a bunch of drive bays or a smaller micro ATX type case, so knowing the case model is imperative.