Disney could charge a premium (above what they already do) for a once a year Adult Only Sailing. Set it in Early Sept (mostly already adults on those cruises anyway) and it will sell out before it hits Gold Booking day.
Things they would need to offer to make it really work-
7 Nights or Longer
A Disney Guest Speaker (Two would be better)
Character Meets (maybe rare ones) held in the Kids Club daily
Events held in the kids club like crafts
Music Trivia being held in at same time, same questions in two locations (like D Lounge and Evolution\Tube)
Match Your Mate in held in the theater, and this might also work well for games like Pop.
Use the Vibe\Edge for Video Games tournaments (I know many adults who love video games)
Give the adult only sailings a cute name- Make merch but keep years off of it, sell it annually on the sailing each year until stock is gone
Offer an adult only afternoon tea (maybe with characters, maybe without) for an up-charge
Offer BBB adult only makeovers (with a special name to make it clear only for this one sailing) for an up-charge
And there is a 0% chance
DCL would lose money on it. In fact they could offer more drink tastings and I think adults on an adult only cruise would spend more on alcohol purchases than what DCL would potentially lose out on kid's onboard merch (and even then adults will still buy stuff for the kids in their lives)
I honestly cannot believe DCL hasn't done it yet.