Agile and Scrum start with the assumption that a team is sufficient for the task set before it.
This is a bit wacky unless we allow the truth, which is that humans are very inventive.
Thus, a given 7-member Scrum team can do many things to gain success:
* change the team
* get other impediments removed
* work with the Product Owner and maybe customers to redefine what is wanted
So, the idea is only common sense. By yourself, you have some power but it is limited. But 7 people, believing in themselves, can do almost anything. If they believe in themselves, they can be almost irresistible. They can reinforce each others' resolve. They can find new resources. They can redefine the problem.
Now, is every team always irresistible? No, not if they do not believe in their mission.
So, Agile and Scrum presume that the doctrine of sufficiency applies. It does not assert that that must always be true, but rather that that is the best going-in assumption.
They assume that by taking action, we can make our lives better. Rather positive, no?