~snip~
Your logic works when one or two countries are hosting the world cup. But it doesn't work when a large group of countries are co-hosting this tournament. And in 2030 it is going to be even worse. There is a combined bid from Uruguay–Argentina–Chile–Paraguay. If they succeed, then there will be 4 co-hosts and each one will qualify for the tournament automatically. And for all the other teams from CONMEBOL (including Brazil), only 2 spots will be made available for the 2030 World Cup. Do you really believe that this is fair?
Did they change that rule?
Because in the past even if the host qualified, for example Brazil 2014, South America still had their full 4.5 slots, even though Brazil was qualified as host.
In that world cup, on top of Brazil, these teams from South America qualified: Argentina, Colombia, Chile, Ecuador, and Uruguay went on to play against Jordan and qualified. So, overall, there were 6 teams from South America in the 2014 world cup.