Whats the benefits of changing torsion bars
Springs get older and weaker with age and duty cycles. Period!
There are no "wonder springs" that last forever. Everything fatigues and gets weaker. This is why rear leaf springs sag with age.
We had many broken torsion bars at the shop where I was a mechanic in college where they had broken. They were used to do front end inspections and look for loose parts before being aligned. A torsion bar is just a bar of steel used as a spring.
Comparing a leaf spring to a torsion bar isn't an equal comparison, apples to oranges. Completely different design and function.
As far as getting "weaker", they actually don't. The effective spring rate remains THE SAME over the life of the torsion bar. It's just physics.
They do, however, sag. The bars settle over time, and although the spring rate remains the same, the car loses ride height. With torsion bars as old as the originals, it may even get to the point that the torsion bar adjusters aren't able to compensate. But the car sitting on the bumpstops isn't because the bars got
weaker, its because the indexing has sagged.
And of course, damaged springs fail. They should not fail from use, as they never exceed their elastic limit in the design of the suspension (as was already pointed out). But, a little nick or chip can cause a failure, and I'd bet that the failed bars that you saw had damage to them (not wear).
Regardless, you should change the original bars. The original spring rates and alignment specs were based on hockey puck hard bias ply tires. Even just switching to old design radials, like the BFG T/A, should mean an upgrade in spring rate and alignment.
Changing to a modern design/compound tire really demands a change in torsion bars, the change in traction is HUGE. Modern cars being able to pull 1 lateral G on the skidpad has almost everything to do with the traction coefficient of modern compound tires, and very little to do with changes in actual suspension design. That increase in traction increases the loads transfered to the suspension because the tires are gripping instead of sliding, which requires the use of larger spring rates to manage the increased loads. It also begs an improvement in stopping power, as improved traction coefficients mean that the little drum brakes will no longer lock up the wheels.
1" torsion bars are a very nice upgrade to most cars, I ran a set on my 318 Duster for a bit. I found them too soft still myself, and have since upgraded again to 1.12" bars. Those won't be for everyone, as they will provide a very stiff ride. But not everyone runs 275 wide tires front and back either, and the additional spring rate comes in handy when managing that kind of traction.