If the platypus and other monotremes went extinct, would the crown group of mammals be redefined? Would mammals then have said to evolve later than their current estimate? Would platypus then be extinct non-mammals?

Maybe I misunderstand something and it won't be redefined, but it's still pretty absurd: if an animal was extinct the year before a group was defined, it would be potentially excluded from the group?

Maybe scientists should stop using crown groups all together?