Diesels like this are not going to go anywhere near 20K hours, or even 20K gallons of fuel. Sorry to pick on you, but I get rattled any time I hear somebody reference the diesel life span myth. It once might have been true on a workboat, not a recreational boat that sits, but modern turbo diesels are not the same animal. Good engines, sure, but still lets not compare to an old school naturally aspirated engine that is not asked to deliver close to its potential, and is also in a setting that gets daily attention.
Biggest issue with Volvo? If the dealer decides Volvo is not supporting them, as a customer Volvo will not support you and the customer gets screwed with full knowledge of Volvo. For that reason alone I would never own one again. I think the engineering is cool, but best admired on someone else's boat without the personal risk.