The data that has been collected over the time since 'Exposure' was first introduced to regulations (circa 1999), indicate that that reliability has improved slightly - but
not by an order of magnitude. When the data was first analysed for the regulatory risk assessment, it was believed that a number of engine/helicopter combinations would not be able to achieve 1 x 10**-5. In the event, most have. That could have been as a result of control and monitoring systems such as FADEC or UMS.
It would not be appropriate to lose sight of the objective of Category A certification stated in the definition:
Category A: With respect to helicopters, means a multi-engined helicopter designed with engine and system isolation features specified in Annex 8, Part IVB, and capable of operations using take-off and landing data scheduled under a critical engine failure concept which assures adequate designated surface area and adequate performance capability for continued safe flight or safe rejected take-off.
Category A departure and arrival procedures are deterministic - i.e. if there is a failure, safe flight can be continued. If engines ever reach a reliability figure approaching 1 x 10**-9, a rethink might be appropriate. (Under the current regulations, if OEI, HOGE is achievable, no additional procedures are required.)
There is already a great deal of flexibility in the system even with the current reliability status of engines. If a State wishes to have a 'Code of Performance' where failure resulting in a 'Hazardous' or 'Catastrophic' outcome has been
risk assessed (to a defined safety target) and found to be 'tolerable' to its population, and those taking part in the CAT activity, that can be done (and is ICAO compliant). However, for those who are not taking part in the activity - i.e. third parties (including property), or for a higher number of passengers (greater than 19) - the current standards of Category A are usually applicable.
The whole regulation of aviation certification and operations is based upon risk assessment (the targets are set down in regulations and guidance). Controls are only in place where failure results in an unacceptable consequence
to some defined degree. Yes there should be flexibility and adjustment of the system when societal mores change or when necessity dictates - but that is already the case.
JimL