Argentina is not gonna get their shit together by next year. They don't have the talent in the roster nor the foresight to do so. If Arthur becomes a very good player for us it's game over for anyone but Brazil in the Copa, only MAYBE Colombia could challenge them.
Messi retiring now means he gets a rest every single international break of the season. That gives room for us to to tell him play harder and more active when he does play, rest all the international breaks, and rest some easy games here or there when the CL knockouts come around. It could extend his prime for Barca by a season or two doing things this way.