Just returned from a week long trip and noticed one of my 2009 D rated Marathons on our 8500~lb 5th wheel has finally started to grow in diameter on one side. Six years is past what I feel comfortable with with any tire LT or ST so in my humble opinion, I got my money's worth out of them. I'm probably going to buy another set of Marathons for it.
Our previous 5th wheel didn't do well with D rated Marathons. At 10,400 lbs, it chewed them up in about three years. I've never had a blowout as I'm fairly obsessed with checking not only pressure but any signs of trouble. Usually starts with a rib groove just a tad wider at some point around the tire's tread. Hard to see unless you're looking for it.
It's been my experience that up to about 80% of the tire's rated capacity works fine for ST tires for around 5 years if cared for properly. I don't expect any more years out of LT tires on the truck but I wear them out in three to four years so I'll never know. We watched an LT tire tear up the side of an older Hitchhiker 5th wheel on I-10 in the Florida panhandle a while back. They get old and give up too. They just cost more and do more damage when they let go.
U-Haul has gone back to bias ply tires on their much abused trailers. I can't help but believe they're on to something. When they come apart they don't tear up everything around them.