Untrue, as has been demonstrated in previous threads on the subject.
There is no evidence that their disappearance in Asia and North America was due to competition from placentals. In addition, recent evidence indicates placentals may have been present in Australia before the arrival of marsupials (Ausktribosphenos nyktos and Bishops whitmorei in the early Cretaceous) or about the same time as they arrived (Tingamarra porterorum in the early Tertiary). If so, one could just as well argue that marsupials outcompeted placentals in Australia and caused their extinction there.
This is flat-out wrong, as has previously been pointed out to you (but you continue to repeat it anyway). The borhyaenids became extinct *before *the arrival of most placental carnivores after the Panama land bridge was complete. They were not wiped by placentals.
No, it’s not the “one single exception.” It’s the only species that colonized temperate North America, but another 13 species of opossums occur in tropical North America. Many of them are common and successful and do fine in the presence of placentals.
Besides this, opossums are very diverse and successful in South America despite the presence of many placentals. There are more than 110 species that fill a variety of niches, from generalized omnivore like the Virginian opossum, to insectivores, frugivores, carnivores, and even the fish-eating Water Opossum. There’s no evidence that placentals are superior competitors to these species.
As for why the Virginia Opossum is the only species that has colonized the temperate zone, it’s probably tougher for groups of tropical origin to colonize the temperate zone than vice versa. Placentals of South American origin have shown no better ability to colonize temperate North America - the only contemporary ones being the Nine-banded Armadillo and the North American Porcupine. The placental New World monkeys have failed completely to colonize the temperate zone.
Many South American marsupials live in temperate conditions in southern South America and the Andes, so it’s not that they as a group lack the ability to adapt. It’s just that the potential colonists in northern South America and Central American are tropical species.