Torsion axles are supported on polyurethane rubber rods in the axle
housing. It is the compression of the rubber that gives the wheel it's "suspension". That rubber wll permanently compress over time allowing the trailer to sink and the suspension to bottom over bumps. If the weight
is substantially removed from the axle when the trailer is not in use, that axle could last a very long time. The problem is most people just park them and forget them.
There are so many advantages to torsion axles! They allow for greater center axle clearance because they use a swing arm to the wheel. They are also self dampening to a point. They do no not produce the violent rebound after an impact that springs do. They protect the structure of the trailer from shock, as well as the contents. They produce a true independant suspension allowing better control over obstacles.
Leaf springs sag over time as well! As they age they need regular inspection because they can fatigue and crack. A failed leaf spring can result in a serious accident, so I'm not sure I would run a leaf axle any longer than a torsion on something like a travel trailer. I don't even like them on utility trailers because they bounce so badly.
Replacing a torsion axle is barely a sixpack worth of work, and worth the money too IMO.