If its a 20,000 hour machine, it probably has round dials, 3 axis autopilot, small windows, cramped cabin with poor escape paths, no EGPWS, no TACS, separate weather radar, no FADEC, no FLI etc. It was probably certified to a lower standard, is not flaw damage tolerant, has no aux lubrication system (which not all do now) or run dry capability.
It is also probably difficult to support and has poor payload compared to the newer types.