@Alfista
Unfortunately, EconomyManager is no longer supported. This is due to the fact that there is an EconomyService API PR that will be hopefully accepted soon. Until then, there are multiple Economy plugins with an API. For now, I recommend using EconomyLite or TotalEconomy.
As for creating one economy plugin, I don’t believe that that is a good idea. Competition is a good thing, it pushes developers to continue to update and improve their plugins. This bring features that never would have happened with only one economy plugin around.