Had the Kings somehow acquired Marner, the team certainly would have needed more help to compete for the Stanley Cup. Marner was never the be-all-and-end-all solution. I agree that the organization likely wouldn't have gone after Panarin if Marner was on the team, but can you imagine having both?
Part of the thinking in acquiring Marner was also setting up an environment that would appeal to Connor McDavid should he elect for free agency after the 2027-28 season. The two displayed great chemistry playing together for Team Canada.
The drawback would be GM Ken Holland himself. Why would McDavid leave the Oilers, whom Holland help position to go nowhere in the postseason with lacking rosters over the years, particularly at goaltending, only to join another team managed by Holland with question marks at goalie as well?