The tread looks much better than the mileage would indicate.  The 4,200 miles of loaded touring was split about 50/50 between the rear and front tires, and the 10,000 miles of "put out to pasture" duty has all been on the front wheel.  The Panaracer Tourgard Plus also has the inside flat-resistant layer (that's what the "Plus" designation means), and none of it is showing at this time.  

Which reminds me of one of my favorite stories, as told to me by Bob Anderson from New England years ago at a B&B in Carlton, Oregon.  He and his wife were biking through the Snake River Canyon along the Idaho/Washington border and flattened on a goat's head thorn on an empty, unpaved road miles from nowhere

He tried the $1 trick also, or at least as best he could given that all he had was twennys.  He reinflated the tire, there was explosion, and the sky filled with $20 confetti.  He carefully collected and preserved all the shrapnel, and when he got home he reassembled the bill, took it to a bank, and they replaced it.

it looks like I take a different approach than most folks, especially now that we are normally on the tour and out of the country for months at a time.  It's really worth a lot to me to minimize the risk that we'll flatten in terrible or unpleasant circumstances, so we start most of our extended tours by replacing any tire that looks significantly worn, even if it looks like it could easily have another 10K miles of life in it.  On the scale of the cost of the tour as a whole, the wasted expense is a nit.

Scott that is sound advice for anyone commencing a long tour.

That's exactly what I do, and I also start with new inner tubes as well.  Whether the tire looks worn or not, I almost always start a long tour with new tires - the only exception being those rare times when you just can't get new tires.  I note the mileage on the used tires and put them in a queue to be used on my daily rider/training bike, and then ride them till they wear out.