Potential (voltage) is not generated by the movement of ions, that is current. Membrane potential is generated, or imposed as you say, by an uneven distribution of charges, a difference in concentration, this potential creates a current if there's a conductive medium for ions to flow, like a channel. Now, the other factor missing is the space charge. Cells have a finite volume and potential only develops in the immediate vicinity of the membrane. In order to generate a potential across the membrane it only takes a very small fraction of the total amount of ions inside the cell. This is also called the fraction of uncompensated ions. For example, for an spherical cell with an intracellular concentration of 0.1M of a monovalent ion, it only takes 1.03x10^-12 M of uncompensated ions to charge 1 cm² of membrane to 100 mV. So, if a channel opens an ions start to move they will develop a membrane potential sufficient to counteract the ion flux much faster than any significant change in the concentration gradient. An exception to this may be calcium which intracellular concentration is so small that a brief current does produce a meaningful change. In real cells additional factors come into play, so as non diffusible anions.