What a load of BS?
If the joint bid from Uruguay–Argentina–Chile–Paraguay wins the hosting rights for FIFA World Cup 2030, then a total of only 6 teams will participate in that tournament from the CONMEBOL region. Out of the 48 participants, 6 are supposed to come from this confederation. And if they win hosting rights, then 4 out of the 6 slots will be reserved for the hosts, while the non-hosting countries within CONMEBOL will participate in the qualifier tournament from which the remaining 2 slots will be decided. So let's be clear about it. A total of 6 teams will be from CONMEBOL, and not 10 as you are claiming.
I went back to read the official rules. Things have changed!
In Korea-Japan 2002, there were 3 countries that qualified automatically: South Korea and Japan as hosts, and France as the current champion (they won France 1998).
In the past, when a country won the world cup, they qualified automatically for the next world cup, but that is no longer the case:
Brazil ended up winning the Korea-Japan 2002 tournament, but they didn't qualify directly for the 2006 world cup(they still qualified through the normal channel though).
Also, there used to be an extra qualifying slot for the host country. For example, Brazil hosted the 2014 World Cup, so they qualified directly.
So, for the 2014 World Cup CONMEBOL had 4.5 slots, but 6 countries qualified. Brazil as hosts, Argentina, Colombia, Ecuador, and Chile as directly qualified in the tournament, and Uruguay after the playoffs.
Until now, the hosting country used to get an extra slot, but for the next world cup this will change:
So, yeah, theoretically speaking, if they have the 4 countries in CONMEBOL hosting the 2030 world cup, they would only have 2 direct slots left (and an extra one for playoffs for a maximum total of 7 countries qualified). Although it would be a strange qualifier then, I'm sure they will change the rules again by that time, as they keep doing it.