The problem with that argument is this -

1) Precedent is not necessarily an indicator of what current intentions are.

2) Argentina should never have been awarded co hosting rights originally for 2021, because it went against CONMEBOL's rotation of alphabetical order, and they had already hosted in 2011. That plan fell through and Brazil are hosts again, but still, they tried to give it to the Argies way too soon.

3) CONMEBOL fluctuates the years Copa Americas are held in because they want the revenue from holding these competitions. It's all about money. And the format of the 2021 edition looks deliberately set up so that Argentina and Brazil could avoid each other until the final. In order for them to have meet earlier, one of them would have had to finish third in the Group or lower.