I run quite a number of Seasonic X PSU's, oldest is from 2009 and still running. Some of the PSU's are in home servers. Jonny Guru one said the quality on the Seasonic X units is really server grade.
I do have a Seasonic G that needs RMA ing, but that's what happens when you cheap out on a G series and not the X series.
Back to the question, I think any electronics that's built correctly can last many years. I have Pioneer amplifiers over 25 years old still running, they of course have PSU's in them.