How is it that the performance of a Torbeck valve can gradually degrade
over time, and yet when the time comes to look at it, nothing less than
a full strip-down and service will do?

Surely it should be possible to just back-track a little to the point where
it was not failing?

But why bother? Toilet with Torbeck valve requires attention every year.

But downstairs toilet (qv) with traditional ball-valve has given
trouble-free operation
for over 10 years.