CliffordK

My old Cinelli handlebars are probably over 40 years old, and have had a lot of miles. And, worked hard doing high gears up hills. They've been moved from my road bike to the cargo bike. But still are in use. Perhaps I'll get another 40 years out of them.

The flat bars that the OP had are a little wider than the curved bars creating different stresses, so that may be an issue.

Here is an interesting short film on handlebar production.
Pinkbike Exclusive Video: How It's Made - Handlebars - Pinkbike

I suppose "low tech" in the production methods, but still more complicated than I had imagined.

I wonder, however, if the raised section in the middle of the old Cinelli bars was made differently. In particular, perhaps it was cut on a lathe and was left full-thickness, whereas the mandrel (or hydro) forming might leave a thinner central section, and at greater risk of fractures.