Every biography I've read so far says that by the time of Muhammad's (pbuh) birth, the fortunes of his clan, the Bani Hashim, had declined considerably. But I'm confused about the cause of this decline, what exactly decline means in this case, and especially the reasons for it to have happened so rapidly. Some details on why I'm confused:
In his book "Muhammad: His Life Based on The Earliest Sources", Martin Lings goes over some of the history of the Quraysh leading up to Muhammad's birth. Lings reports that Hashim, son of Abd Manaf bin Qusayy, was granted the right of caring for pilgrims and levying the tax, and also that Hashim was held in high honor.
Lings goes on to say that when Hashim's son Abd al-Muttalib (Muhammad's grandfather) rediscovered the well of Zamzam, a share of the treasure buried there went to Abd al-Muttalib, and the other clans of Quraysh ceded the rights over Zamzam to the Bani Hashim.
This seems like an excellent position of wealth -- and especially political power -- for the Bani Hashim. How did the clan fall from such a height by Muhammad's day, even while Abd al-Muttalib was still alive? What does it mean in particular that the Bani Hashim had declined? Had they used up all their money? Had they somehow lost all their political power? What could have caused such a catastrophic decline?
Edit: perhaps the decline occurred because Abd al-Muttalib was extremely generous with charity and hospitality. If that is the case, then I wonder: did the customs in those days prevent generous people from investing some of their wealth to grow it so they could be even more charitable and hospitable to more people? Or another way to ask: would the rules of charity and hospitality require people to bankrupt themselves?