If you opened an ECU in your car, you could resolder a failed cap. There is nothing preventing you from doing that. The repairs it might prevent would be if you replaced an ECU that needs to be authorized to communicate over the bus. I can't speak for other automakers but at least in our case you don't have to go to a dealership if you don't want, there are third-party repair shops that are authorized to use our software for these kinds of repairs.