1) you would do yes, but most people will run it on a raspberry pi so running costs would be £5-10 a year.
2) Yes I'd agree with your first statements. It's quite a learning curve to start with, but once you're familiar with config/setup then it's much easier. As for easier to control/better interface, that's down to the individual to decide on layout/how the lights are controlled.
3) Yes definitely more quicker than opening up individual apps. You could set up buttons for lights and sockets on the same page if needed.
Where it also comes into its element is being able to integrate various manufacturers. I.e. You've bought gosund now, but next year when you decide you want more smart plugs, they may no longer exist, or you'll find a better deal on another brand. All adding to the nightmare of having to open up separate apps.
Your analogy is fair. I guess the best way I can sell it is...
A) It's inevitable that you'll buy more and more smart home tech, and it will become tedious having manufacturer specific apps to control separate devices.
B) HA is completely open source, so you'll never get cornered/blackmailed into paying to use it. Have a look at the stories about Wink from a couple of years ago, where users had purchased the device and smart components, and Wink blackmailed their users into paying a monthly fee or the devices would become bricked.
