For pleasure boating, I think the draw to a diesel outboard is about using a single fuel, and getting rid of gas on the boat, and the jugs or storage tank associated with it. It's a big nuisance that we all just put up with because we have to. I would love to get rid of it, and would put up with some extra weight and cost to do so.
I also think there is an interesting possible fit for small trawlers - say in the high 20' through 30s range where displacement speed power requirements are modest, and space is at a premium. Making some of those boats outboards could be a big win, and I doubt weight is a big issue for them.
All that said, I'd want to see how reliability and durability really shakes out. A big part of why a diesel lasts longer is because they are slow moving. For every mile you travel, all the moving parts drag across each other for hundreds, if not thousands of miles of wear contact, and it's directly proportional to the engine speed while traveling. Add in the higher cylinder pressure of a high-output engine, and wear just goes up more. To fit a diesel into an outboard package, you need to do all the tricks to the engine design that reduce size and weight, but also reduce longevity. So I don't think it's a given that one of these diesels will last a lot longer than a gas outboard. I certainly wouldn't presume it as part of a purchase decision.