Lower fuel costs, greater durability, higher torque, no cavitation, no blower required, a real alternator, no carburetor to varnish up, spark plugs to foul, coils to die, or ethanol eaten fuel lines to worry about, and if one keeps up on the minimal maintenance required, you never have to worry about how easy it is to swap out.
Lots of sailboats out there with reliable 30 year old diesels still chugging away- not many with reliable 30 year old 2 stroke outboards.
Some of what you say is germane, some not. Lower fuel costs? Diesel costs more than Regular. Durability? I'll give that to diesels. Cavitatiion... I'm speaking of purpose-build outboard wells... no cavitation on my boat. No blower, OK, diesels have a $100 advantage. Real alternator... any boat in the outboard-sized realm probably does not care much about an alternator. A 30 watt solar panel costs $60 on EBay.
Carb to varnish up? Ethonol-free gas is available if one searches. If one must, carb cleaning is a 2-hour job. Plugs to foul... $10 spares and 5 minutes handle that. Ethanol eaten lines... one can find ethanol-free gas in many places... do a search. I have not run gas with ethanol in years.
I am not saying one is better than another, both have their merits. But when an outboard fails, it is a 30 minute change-out. With a diesel, it is 15 days... and counting.
I think it is more instructive to discuss pros and cons rather that categorically state one is better than another.