The Persian empire was successful is because of their tolerance of non Persian citizens living in Persia. Persia also conquered many places making it successful.
Decline of rural living, but sparked tremendous amounts of economic growth eventually. Early urbanization was filthy though, no proper sewage, overcrowded cities. So many people surged into these urban areas at one time, often not finding enough jobs for everyone.
Social discord, political instability and lack of compromise, economic collapse.
It's not the broadest: the broadest is M3. Also, it's not true that it consists of assets that are not liquid.
The best answer is that it represents money that people have easy and immediate access to
