Psu's either work or they don't, there's no inbetween. If it's functioning as it should, as it's claimed to be, that's good, it's when it doesn't, if an output is out of spec, if a rail doesn't put out what it's supposed to, it's done, dead in the water, whether it runs or not.
The Seasonic GB platform, the base for the S12-II, M12-II, XFX TS, Antec HCG-M, 520w-550w-620w and some others is a workhorse. At its inception 10 years ago, it was as good as it got for a Bronze rated unit and better than many Golds. Even today, it's still a workhorse, and still in production, even though it will no longer win any performance awards. Seasonic said 'this is what you get' and that's what you got.
Are there better? Yep, sure are, even some that are not only better, but cheaper too, but that's also market depending. Definitely in the US, Europe, and the other larger variety markets, but in the backwoods on many Indo-Asiatic markets, anything built on that platform is still about the best and most reliable you'll get in that budget range.
I just sold my 7yr old M12-II, I tested it before letting it go, all rails still at the same output as they should be, purring like a kitten and never once a blink or worry from the psu. It ran 24/7/365 on OC 3570k and OC gtx660ti.
Replacing the psu is preventative maintenance. Tires are getting bald, whether they still hold air or not. It's an age and wear and tear thing. You are looking at a psu that came with a 3yr warranty, you got your money's worth, and then some. Still got a few miles left on them tires, but replacement is definitely a consideration now, sooner or later. At next upgrade for sure.