Shedding effectivity depends on the amount of unspent outputs. E.g. If there are 1.5M transactions, and 1.2M addresses still containing money, replacing transac

Shedding effectivity depends on the amount of unspent outputs. E.g. If there are 1.5M transactions, and 1.2M addresses still containing money, replacing transactions with account ballances won't give you much. As for the environmental concern - read up about the tragedy of commons. Few people will abandon a better protocol into a worse one if the only benefit is ecological.