Messi needs to win the World Cup to complete his legacy. I wouldn't think any less of him if he didn't but there's just something missing if he didn't. Champions league is just a competition for the richest club.
I was amazed he was able to get that Argentina side to the finals and he was out of gas when they got there. Like winning the Super Bowl he will have a hole in his resume of they don't win a WC but it really won't diminish his greatness as a player it will just make it difficult to be considered the GOAT.