Wayne, on one hand, yes you got lucky. On the other, you got kinda lucky.
There's real truth to the replace every 3-5 years theory. (3 years is excessive IMO, but there's actually a guy on here who changes his trans fluid every 500 miles too, so....)
I've had china bomb trailer tires last as long as your's. I've had 4 year old tires blow treads off them. Have had 10 year old tires that worked fine still, got beat on and then blew up in the driveway.
Bottom line, I personally wouldn't go more than 5 years, but loading, wear, care, heat all play a part in tire life. Knock on wood, I've never actually lost a tire in the winter when weather was cold and I tow almost as much in the winter some years as summer.
8 years, 30k, yeah buy some new ones. FWIW, I have the same tires as you on both of our tandem axle trailers. 14" Rims. Kenda Karriers have dun gud so far. I'd buy Karriers again based on my experience with them thus far.