Originally Australia and Qatar were to play in the tourney as well, so would have been 5 games to knock out 4 teams... still oddball, but hey, Some Euro groups see three of four teams advance.
Copa <> Conmebol. Two different entities. USA hosted the Copa America Centennial in 2016, finishing fourth and had a bid to host what would have been 2020. I love seeing us playing South American teams and hope to see more integration.
As far as a merger goes, pretty sure it's all of CONCACAF, not just the USA that wouldn't want to combine for World Cup qualifications. Even Mexico would have to work to qualify, let alone the other two or three teams who have a shot at making at least the World Cup or that play-in match.
And travel would be an absolute nightmare. I'm currently in southern Brazil, and I am closer to Cape Town , South Africa than Miami, the closest point in the US. For comparison, Moscow is 2,500km as the crow flies from London. 2,500 miles due North wouldn't even get me out of Brazilian air space. Curitiba to Manaus is 2,700km and change.
Either Buenos Aires or Santiago to Toronto is over 8,500km. Basically, pretty much the same as London to Vladivostok, Russia out on the Sea of Japan.