If you look at the quality of the players in Argentina and Brazil, these two teams could actually dominate the Copa America this season. Because Argentina also has talented young players, the Argentine side will definitely not rely too much on Leo even though Messi himself is played as the main player in the Argentine squad. However, the Brazilian players also have more young players who are quite dangerous so it will be quite interesting when these two countries face each other in the Copa America match in certain rounds.
I dont think Brazil with their current performance will be able to dominate the Copa America, Brazil performance in the FIFA WCQL last year was very bad, from 6 matches they could only win 2 times and lose 3 times.
Brazil also just appointed their new coach in January this year, 5 months perhaps not enough to bring Brazil performance back to the top.
While Argentina, after becoming World Cup Champions, have won 13 of their last 14 matches in the FIFA WCQL and international friendlies, Argentina will clearly dominate the Copa America this year, not Brazil.