"Old mechanical engines don't like being driven at 20% engine load, as the OP stated."
YES BUT, most older engines are tolerant of long time under loading , as bulldozer engines idiling and light loading were daily norms..
AS truck engines many were idled for 8-12 hours every time they did a long trip.
Yes,, the longest engine life is with the engine powering a generator head , constant load , only a very few cold starts , oil changed on the fly , and proper diesel coolant.
SO WHAT ???, the engine that might run 12,000 hours in this ideal setting might loose 20% or 25% of its service life with too light loading, sitting months , cold starts , and a constant damp caused by the wet exhaust.
Most owner change oil more frequently than required (no oil analysis) which helps get rid of the acids from blowby from light loading.
So that 12,000 engine becomes a 10,000 hr engine before the oil consumption gets too high.
For most boats 10,000 hours , at the common white boat 200 hours a year is......FIFTY YEARS!!!
I would never swop an engine with many decades of service life left for a tiny car engine , even in theory that it should be "more efficient".
An old engine goes out to anchor service when it is less work to swop it out than find parts for.
Makes me a Detroit 6-71 fan , but there are lots of aftermarket items for most Cats too.