However, if you consider systems A and B, where system A is a bit better at using immense power for good than system B, you will notice that system A will result in a lot more good than system B. Therefore, if you care even a little bit about good things happening, you will be very keen to have system A instead of system B.
The fact that someone with immense power does something good becomes a lot less reassuring when you consider the opportunity cost of not having someone or some people who are a lot better at achieving good have that immense power.