The new aircraft are never going to make money in charter and the Owner can live with that.
This has been the problem with GA for many years, even when the C310 and PA31 were brand new machines. GA has for a long time been a hobby farm which has capped its own potential more than any over-bearing regulator. If you look where the new aircraft are going its in the well run areas of GA like the many new large budget flying colleges, aeromedical services etc...
Charter companies in GA are generally very poorly managed small operators with the wrong equipment. How many successful bus and taxi companies operate one vehicle, not many because to offer reasonable fares with reduced overheads you need a number for satisfactory income. Aviation is no different, you need a number of aircraft and pilots with constant utilisation to make it work whether its a school, charter or RPT.
In the US they have the jet share programs for private operators and so on.
The problem is the aircraft gets hamstrung with training, crewing and over attention by casa. Eventually the owners get jack of it and gives charter a miss. The punters are then left with the old 20 - 35 year old aircraft again.
What punters are we talking about here?
Those that can afford a jet can charter them from the established jet operators, there are a number around each major city. Unfortunately a new jet is not going to compete on a cost basis with an old piston aircraft, even if you mandated the same T & C schedule. However most "punters" that can afford regular charters will stipulate turbine only and will pay the premium.
As Mach E Avelli stated earlier once you get over the lack of performance in an engine failure generally if the older aircraft are operated and maintained to the rules they are easier to keep current on.
The jet however is hurtling around in the flight levels with all sorts of other considerations other than engine failure to contend with.