I have a tractor that has tires that are probably 40 years old. Still working great. Of course I don't get them hot with high speeds.
I agree. We are successfully using many very old tires on various equipment. Loading and speed play a great part in tire failure regardless of age.
Actually most of the modern trailer tires are going to radials. Much better ride and handling characteristics.
I've not found this to be the case. Bias ply and radial tires each have their uses. Tires designed for trailers in my experience continue to be bias ply for several reasons. The stiffer sidewall of a bias ply tire leads to greater load bearing capacity and sway resistance. A modern radial tire has a more compliant sidewall, contributing to a more comfortable ride, and preferred handling characteristics for an automobile. A quick bit of Google research turns up the following:
Trailer Tire requirements differ greatly from automotive tires. Automotive tires must maintain traction during all driving conditions: pulling, stopping, turning, or swerving. Because of this they must have more flexible sidewalls to maintain tread to road contact. Since trailers have no driving torque applied to their axles, the only time trailer tires must have traction is during the application of trailer brakes.
Trailers with heavy loads, high vertical sideloads (like camper trailers), or trailers with inadequate tongue weight can be affected by trailer sway problems. Automotive bias or radial tires with their more flexible sidewalls can accentuate trailer sway problems, whereas the stiffer sidewalls of the ST (special trailer) bias ply tires help to control and reduce sway problems. For this reason it is not recommended that (P) Passenger or (LT) Light Truck tires be used on trailers. Best trailer control will be achieved with (ST) Special Trailer tires.
That's not to say that passenger radials cannot be used on trailers. Indeed, we have several trailers all with radial tires. In reality, with the weight range we are dealing with in teardop trailers, most any tire in good condition will suffice. To each their own...that's what makes the teardrop community great. There are about a million different ways to do any given thing and we're bound and determined to try them all!

Brad