Brian Gaff has brought this to us :
As a matter of interest the hoses with that metal kind of spiral outside,
what makes them start to get stiff? Is it our old friend the scaling up?
I guess this means that the membrane the pipe is made from inside is
probably not long for the world.

Good question, my guess is scaling up on the outside makes them stiffer
- though mine seems to become less stiff once it is warmed up by the
flow of water. I don't recall ever having one leak.