I think the reason why they were able to win against England is that the England team is not in the best state right now and West Indies were playing on their home ground. but the situation is absolutely different when they will be playing against Australia and South Africa.
But the teams that are at the bottom are going to perform well for only so much time right?
I think it is just a matter of time before these teams are going to face harder opponents and ultimately lose against them. and on the home ground, these teams have some certain advantages so they are playing well. I don't think in other conditions they will be able to repeat these types of results anymore.