He's absolutely worth it to the Dodgers if they finally win the World Series. Like you said, Betts will be considered the last piece to get them to promised land, and none of us -- even those armed with data -- will be able to prove he wasn't. A player's value to a club in the dugout, clubhouse and airplanes can't be quantified, and even if such settings are compromised this summer, there's also a confidence factor that is underrated when a new star joins a club and takes the pressure off of teammates.
History is rife with acquired newcomers who put teams over the top, but it's usually pitchers -- Verlander last decade, Lidge in the 00s, Maddux in the 90s, Willie Hernandez in the 80s. But there are also examples of superstar regulars turning good teams into great teams: Joe Morgan in the 70s, Frank Robinson in the 60s. Over half those guys are Hall of Famers (though I'm sure there are plenty of moves that have backfired, too).