Is there any definitive proof showing......

I think that "definitive" will be pretty hard to establish for such a broad application as "yacht service" since few yachts operate exactly the same or are maintained the same or even put many hours per year on a propulsion engine. Yacht service doesn't exactly fit the best model for longevity.

However, there are yachts running around with direct reversing diesels that were built in the 1920s. You could say they lasted the longest, that is certainly definitive, but is that what you are looking for?

From an engineering point of view, some of the Italian high performance diesels are magnificent pieces of art and engineering, they are among the most beautiful to look at and and the performance is amazing. But, that kind of performance comes at a cost, and like any thoroughbred, they have a short working life because high output and low weight comes from high BMEPs and piston speeds. Some of the elements that give them such high performance also lead to a requirement for the kind of care and attention that a long lived but stodgy old lump of iron doesn't need.

There very well could be much better engines than the main stream motors that we see in the yachting industry regularly,
But the fact that parts and service availabilities are very strong will out weigh a lot of other factors.

Hear, hear. Maintainability is just as important as reliability. No one has ever built an engine that won't break sooner or later.

Who services Isotta Fraschini marine engines in the Annapolis area? CAT, MTU, MAN, Volvo Penta all have service networks through out the world.

As a broker people ask me all the time which is best, or how long will brand X last and I can not say. There are too many variables. The biggest engine killer is lack of use and then lack of maintenance, and this will affect all makes of engines.