Separation of obligations.
Thread creep maybe but this can certainly be a real issue. Without having a solution to it to advocate here, I encountered a specific example of this conflict at the initial discussion of ETOPs at ICAO.
Airbus and Boeing had built and certified the A300, 757 and 767 for cross-continental routes but then realised they actually had the range for oceanic travel. They plus IATA sought to have ICAO simply erase the "90 minute rule" existing at that time with no additional precautions other than an engine failure rate for which they proposed a very dubious definition - only a "total non-restartable hard failure of the core engine" would count in the reliability stats.
IFALPA was in agreement with the concept of allowing these aircraft to operate but subject to additional restrictions - demonstrated reliability of overall thrust delivery to include precautionary shutdowns due to e.g. loss of oil, additional systems redundancy, better forecast wx at alternates etc. These were set out in papers to the special study group set up by ICAO in 1982.
The evening before the first meeting the ranking official of one of the major authorities present approached me to say they were in a very difficult situation because of they had this dual remit. Their view was that that the "industry" position was grossly unsatisfactory from a public safety viewpoint, but they had been briefed not to advocate new restrictions due to their "support the industry" obligation. Therefore they had come to the Study Group "only to listen" before reaching an official view on the subject.
Unofficially they strongly supported the IFALPA view (as did other XAAs), but felt it needed strengthening data, which was very true. The end result was a discussion which revolved around "if you were to put it to me that X is the case ...... I should find it very hard to refute, and if you were to mention this specific data that you may have found, that would strengthen the argument!" The end result was the ETOPS regs which have worked very well ever since.