Widgeon,
You are right, at this time no manufacturer has extended the TBO of critical components.
There was a case, I think it was a Bristow S 76, when a peak of abnormal vibrations from the MGB was detected by the system. SK did not recognised it - maybe because it was a retrofit - and refused to deliver a new gearbox under the hourly insurance system, and the operator had to pay for a new one.
I think for the IHUMS or equivalent to develop :
first, accuracy and reliability will have to be improved
second, the manufacturers and the regulatory agencies will have to come up with rules or regulations.