A mild hybrid improves fuel economy by about 15%. A full hybrid by about 30%. No need to make theoretically calculations there are plenty of vehicles with standard and hybrid powertrains.
You can replace electronic modules as long as they are manufactured and sold. Automakers only are required to provide spare parts while the vehicle has a warranty. After that they are free to discontinue them. Popular models will get aftermarket support but with low volume models or even low volume trims you may be out of luck