Yes. But all surviving monotremes live in and around the water, in places where they mostly compete with marsupials. Marsupials have an advantage on dry land.

Yes. But all surviving monotremes live in and around the water, in places where they mostly compete with marsupials. Marsupials have an advantage on dry land. But if a marsupial mother with babies in her pouch swam, she'd drown them. So monotremes have an advantage in the water. Placentals, of course, usually outcompete both. With occasional exceptions, such as the success of opossums in North America.