With oversized generators hydro provides an extra storage that can last weeks and even months at the pick usage depending on reservoir capacity. This is the battery effect that cannot be matched with molten salt.
Increasing capacity at hydro power stations or other baseload stations isn't as easy as just "putting a larger generator in". Everything needs to be considered and sized up: turbines, piping, outlets, new environmental assessments, etc., just as in any other type of power plant.