Recent years have seen a whole audit/safety advisor industry spring up. These firms are mostly operated and staffed with ex airline pilots who bring with them big airline mentalities. Or ex CASA types - enough said! They charge big bucks for their services. Their clients sometimes don't know enough about aviation to question their methodology.
Whatever, to justify their big fees, you can bet that the auditor/advisor will write a bucketload of recommendations none of which will be good for operators of old piston aircraft, no matter how good the individual operator's safety record may be.
The real money is with the turbines, so that is where they want to see the industry go.
If I was looking at some report that meant my charter costs were about to double, I would want hard statistics to prove that existing and cheap aircraft A had double the engine failure or accident rate of proposed and expensive aircraft B - not some weasel generalisation about turbines versus pistons.
If the charter user is taxpayer funded, all the more reason for the recommendation to be backed with hard data.