This is a religious debate in which all parties are not going to agree.
IMHO, a carseat is invaluable--even in business class--because it keeps the baby strapped in and can allow the mother peace of mind to sleep--which will be much needed on such a long trip. Honestly, I am not familiar with PR's cabins but would do some investigation into the seating arrangements in the cabin to see if I were comfortable that there was a way to reliably hem the child in so that it could not wander off without waking the mother.
Yes, the carseat can be a pain to deal with in the airport, but a light one is not a huge deal, and well worth it IME.