Little to compare between Titans / Chancellors / Golden Eagles with Conquests or Caravans - gas turbine power will always vastly improve performance when matched to the same fuselage. Having flown turbine 400s alongside their piston variants I agree I'd rather be in a Conquest or whatever but the cost of purchase / operation of a PT6 or Garret powered machine over a TSIO-540 is prohibitive for most (UK at least) small charter operators, so I still think the 404 / 402 etc. has a role; but as you say at max AUW performance rarely matches that of the handbook when the aircraft is 20 years old.
Where do you stop at banning piston powered light twin charters? Do we keep Aztecs, Senecas, Barons & 310s etc.? Or would you say these were potential disasters as well?
PS when I was discussing taxiing run-ups I really meant a 30 sec stop at the holding point rather than continuing with forward motion; but not to the extent where long noisy power checks need to be endured.