I bought a used 800 watt power supply two years ago. It is still working without any problems. I don't see the point in replacing it if it works properly. Especially if you have been using it for 4 months without any problems, there is most likely nothing wrong with it or the cables.
That is 100% not even remotely accurate. Not in any way, shape or form. Besides which, your sample of one literally has nothing whatsoever to do with another persons experience or outcome. Even if you both had the same power supply model, which you don't, and even if they were both brand new out of the box, you still would be inclined to have entirely different experiences either in the short or long term. Either unit could fail, either prematurely or eventually, due to age, while the other continues working fine. It's no different than with people. You can take any two people, feed them the same diet, same exercise, same environmental conditions, etc., and one gets cancer while the other is perfectly healthy.
It also matters IMMENSELY what the condition of the hardware it's being used with is like AND in fact WHAT hardware is being used with it, not to mention the specific model of the power supply in question. "800 watt power supply" literally means nothing, at all, in any conversation where you are discussing the merits of any given power supply. Neither does it's "80 plus" certification. The ONLY thing that matters is the internal quality and component selection (Assuming the unit has sufficient capacity and can actually sustain that capacity, which is a big assumption with any unit that isn't specifically known to be of at least moderately good quality) which can only be determined by those of us on this end of things by looking at units that have been reviewed with a complete teardown and associated testing by a reputable reviewer using appropriate equipment. Or, to some degree, also units that are known to use the same platform as another unit that HAS been tested and reviewed in such a way, although even units with the same platform that are different models might have wildly different performance results and some internal components might be of different quality, so it's usually best to only rely on actual reviews of a specific model.
The idea that "because it worked for 4 months" has some bearing on whether or not it might be faulty, is, though, is the part that bothers me the most. The fact that a thing was working, in NO way, has anything to do AT ALL with whether it might or might not be the problem. In fact, in 99% of cases, a thing was ALWAYS working fine, before it wasn't, so that is just completely irrelevant to any conversation of whether a thing might be to blame or not.
I will however concede that if the cables themselves were the problem, THAT you'd have known about immediately. Likely there would have been magic smoke in abundance if the cables were not compatible, for any power supply. And the point of replacing a power supply when you are having problems is that if you've got a good reason to suspect the unit, or if you've eliminated everything else as being probable, what you are generally left with is the power supply and motherboard and while it's often a toss up which of those it might be, you can pretty well hedge your bets towards the winning guess if one of them is significantly older than the other. Either way it's a roll of the dice but when you don't know the history of the power supply but you DO know that it's been in service for a fairly long time, it's almost always a no brainer to lean that direction if for no other reason than because it's a high probability type component AND has significant miles on it.
Never make assumptions when it comes to power supplies. A PSU will break you of those assumption making tendencies more often than not. You've been lucky. Most are not.